The intention of this is to allow sources/outputs/etc to be created
without being visible to the UI or save/load functions.

(Note: This commit also modifies UI)

Instead of using signals, use designated callback lists for audio
capture and audio control helpers.  Signals aren't suitable here due to
the fact that signals aren't meant for things that happen every frame or
things that happen every time audio/video is received.  Also prevents
audio from being allocated every time these functions are called due to
the calldata structure.

Transition sources are implemented by registering a source type as
OBS_SOURCE_TYPE_TRANSITION.  They're automatically marked as video
composite sources, and video_render/audio_render callbacks must be set
when registering the source.  get_width and get_height callbacks are
unused for these types of sources, as transitions automatically handle
width/height behind the scenes with the transition settings.

In the video_render callback, the helper function
obs_transition_video_render is used to assist in automatically
processing and rendering the audio.  A render callback is passed to the
function, which in turn passes to/from textures that are automatically
rendered in the back-end.

Similarly, in the audio_render callback, the helper function
obs_transition_audio_render is used to assist in automatically
processing and rendering the audio.  Two mix callbacks are used to
handle how the source/destination sources are mixed together.  To ensure
the best possible quality, audio processing is per-sample.

Transitions can be set to automatically resize, or they can be set to
have a fixed size.  Sources within transitions can be made to scale to
the transition size (with or without aspect ratio), or to not scale
unless they're bigger than the transition.  They can have a specific
alignment within the transition, or they just default to top-left.
These features are implemented for the purpose of extending transitions
to also act as "switch" sources later, where you can switch to/from two
different sources using the transition animation.

Planned (but not yet implemented and lower priority) features:

- "Switch" transitions which allow the ability to switch back and forth
  between two sources with a transitioning animation without discarding
  the references

- Easing options to allow the option to transition with a bezier or
  custom curve

- Manual transitioning to allow the front-end/user to manually control
  the transition offset

Allows using it in multiple source files

(Note: test and UI are also modified by this commit)

API Changed (removed "enum obs_source_type type" parameter):
-------------------------
obs_source_get_display_name
obs_source_create
obs_get_source_output_flags
obs_get_source_defaults
obs_get_source_properties

Removes the "type" parameter from these functions.  The "type" parameter
really doesn't serve much of a purpose being a parameter in any of these
cases, the type is just to indicate what it's used for.

The new audio subsystem fixes two issues:

- First Primary issue it fixes is the ability for parent sources to
  intercept the audio of child sources, and do custom processing on
  them.  The main reason for this was the ability to do custom
  cross-fading in transitions, but it's also useful for things such as
  side-chain effects, applying audio effects to entire scenes, applying
  scene-specific audio filters on sub-sources, and other such
  possibilities.

- The secondary issue that needed fixing was audio buffering.
  Previously, audio buffering was always a fixed buffer size, so it
  would always have exactly a certain number of milliseconds of audio
  buffering (and thus output delay).  Instead, it now dynamically
  increases audio buffering only as necessary, minimizing output delay,
  and removing the need for users to have to worry about an audio
  buffering setting.

The new design makes it so that audio from the leaves of the scene graph
flow to the root nodes, and can be intercepted by parent sources.  Each
audio source handles its own buffering, and each audio tick a specific
number of audio frames are popped from the front of the circular buffer
on each audio source.  Composite sources (such as scenes) can access the
audio for child sources and do custom processing or mixing on that
audio.  Composite sources use the audio_render callback of sources to do
synchronous or deferred audio processing per audio tick.  Things like
scenes now mix audio from their sub-sources.

(Note: This commit breaks libobs compilation.  Skip if bisecting)

Adds a "composite" source type which is used for sources that composite
one or more sub-sources.  The audio_render callback is called for
composite sources to allow those types of sources to do custom
processing of the audio of its sub-sources.

(Note: This commit breaks libobs compilation.  Skip if bisecting)

These variables are considered obsolete and will no longer be needed.

(Note: This commit breaks libobs compilation.  Skip if bisecting)

Removes audio lines and stores the circular buffer for the audio on the
source itself.

(Note: This commit breaks libobs compilation.  Skip if bisecting)

The mixers that a source was assigned to were originally stored in the
audio line.  This will store it in the sources themselves instead.

This has been missing for a bit too long, and should make it
easier/faster to diagnose issues users might be having.

Ensures that the packet dts_usec vals which are generated for
syncing/interleaving use the proper offset relative to where they're
supposed to be starting from.  The negative DTS of a first video packet
could potentially have been applied twice due to this.

Fixes potential race conditions when two threads are trying to
initialize/start/stop the same encoders at the same time.

There shouldn't be any need to call these functions manually.  These
functions are called automatically by obs_save_sources and
obs_load_sources.

Allows transitions to be able to return a blank texture to transitions
if a transition is transitioning from/to a NULL source.

Useful for going traversing the list of audio sources (particularly for
the new audio subsystem)

This was originally used for calculating audio volume if transitions
were active, but transitions won't work that way so tracking the active
transitions is no longer needed.

(Note: This commit breaks UI compilation.  Skip if bisecting)

API Removed:
------------------------
obs_add_source

API Changed:
------------------------
obs_source_remove: Now just marks/signals a source for removal

The concept of "user sources" is flawed: it was something that the
front-end was forced to deal with if it wanted to automate source
saving/loading, and often it had to code around it.  That's not how
saving/loading should work, a front-end should be allowed to manage
lists of sources in the way it explicitly chooses, and it should be able
to choose which sources it wants to save/load.

This prevents encoders (hardware encoders in particular) from being
continually active when all outputs disconnect from an encoder.  This is
mostly just a temporary measure; the encoding interface may need a bit
of a redesign.  It will also definitely needs to be able to flush at
some point.  Currently when an output is stopped, the pending data is
discarded, which needs to be fixed.

Allows objects to be created regardless of whether the actual id exists
or not.  This is a precaution that preserves objects/settings if for
some reason the id was removed for whatever reason (plugin removed, or
hardware encoder that disappeared).  This was already added for sources,
but really needs to be added for other libobs objects as well: outputs,
encoders, services.

This feature allows a user to delay an output (as long as the output
itself supports it).  Needless to say this intended for live streams,
where users may want to delay their streams to prevent stream sniping,
cheating, and other such things.

The design this time was a bit more elaborate, but still simple in
design:  the user can now schedule stops/starts without having to wait
for the stream itself to stop before being able to take any action.
Optionally, they can also forcibly stop stream (and delay) in case
something happens which they might not want to be streamed.

Additionally, a new option was added to preserve stream cutoff point on
disconnections/reconnections, so that if you get disconnected while
streaming, when it reconnects, it will reconnect right at the point
where it left off.  This will probably be quite useful for a number of
applications in addition to regular delay, such as setting the delay to
1 second and then using this feature to minimize, for example, a
critical stream such as a tournament stream from getting any of its
stream data cut off.  However, using this feature will of course cause
the stream data to buffer and increase delay (and memory usage) while
it's in the process of reconnecting.

Allows it to be used in multiple source files.

These functions are intended to log a warning when an obs function is
called with a null object.

Allows the ability to get a module-specific configuration file/path

API Changed:
---------------------------
From:
- bool obs_startup(const char *locale, profiler_name_store_t *store);

To:
- bool obs_startup(const char *locale, const char *module_config_path,
		profiler_name_store_t *store);

Summary:
---------------------------
This allows plugin modules to store plugin-specific configuration data
(rather than only allowing objects to store configuration data).  This
will be useful for things like caching data, for example looking up and
storing ingests from remote (rather than storing locally), or caching
font data (so it doesn't have to build a font cache each time), among
other things.

Also adds a module-specific directory for the UI

Due to all the threads in libobs it wouldn't be safe to make that
parameter reconfigurable after libobs is initialized without adding
even more synchronization. On the other hand, adding a function to set
the name store before calling obs_startup would solve the problem of
passing a name store into libobs, but it can lead to more complicated
semantics for obs_get_profiler_name_store (e.g., should it always return
the current name store even if libobs isn't initialized until someone
calls set_name_store(NULL)? should obs_shutdown call
set_name_store(NULL)? Passing it as obs_startup parameter avoids
these (and hopefully other) potential misunderstandings

(Non-compiling commit: windowless-context branch)

API Changed:
---------------------
Removed functions:
- obs_add_draw_callback
- obs_remove_draw_callback
- obs_resize
- obs_preview_set_enabled
- obs_preview_enabled

Removed member variables from struct obs_video_info:
- window_width
- window_height
- window

Summary:
---------------------
Changes the core libobs API to not be dependent upon a main window/view.
If you wish to draw to a window/view, use an obs_display object to
handle it.

This allows the use of libobs without requiring a window to be present
on the system.  This is also prunes code that had to be needlessly
duplicated to handle the "main" window.

The "clamped" video time is the system time per video frame that is
closest to the current system time, but always divisible by the frame
interval.  For example, if the last frame system timestamp was 1600 and
the new frame is 2500, but the frame interval is 800, then the
"clamped" video time is 2400.

This clamped value is useful to get the relative system time without any
jitter.

Implements exponential backoff for consecutive reconnects, which is
useful to prevent too many connections from trying to reconnect back to
a service at once over a short period of time in the case of potential
service downtime.  Exponential backoff causes each subsequent reconnect
attempt to double its timeout duration.