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

Allows gs_render_sprite to be used without a texture as long as cx/cy
are set.  There's no reason why it shouldn't be able to do this.

Needless function calls here for what should be intrinsic calls

These were in the APPLE section by mistake

Prevents from having to query the base audio subsystem for the sample
rate each time the function is called

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.

Uses 128bit integer math to prevent overflow

Allows some limited 128bit integer math functions, primarily for the
sake of performing calculations on nanosecond-based intervals of time

Before if a source was set to invisible it would still be considered
active.  This changes it so that the source is deactivated when the
source is invisible to reduce needless resource usage or capturing.

Renames:
----------------------------------------
obs_source_add_child
obs_source_remove_child
obs_source_enum_sources
obs_source_enum_tree
obs_source_info::enum_sources

To:
----------------------------------------
obs_source_add_active_child
obs_source_remove_active_child
obs_source_enum_active_sources
obs_source_enum_active_tree
obs_source_info::enum_active_sources

These functions/callbacks had misleading names: they originally implied
any child sources, when they actually meant active child sources that
are being used to render video or audio.  It's important that the
function names represent their actual purpose.

Prunes code used to workaround libobs "user sources" (such as
sourceSceneRefs) and instead simply holds its own references and saves
sources it chooses with obs_save_sources_filtered.

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

Ensures that a reference is kept until the signals associated with that
reference are disconnected.

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

Adds a means of saving specific sources that the front-end chooses,
rather than being forced to use the now-removed "user list".

(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.

These signals are meant to replace the add/remove signals.

These functions created stack variables but never actually initialized
them.  If the calldata variable is invalid, the return values will be
the uninitialized stack value.

The name was inconsistent with the darray_swap function that it's a
macro for.

This function was removed even though the browser plugin was using this
function on mac, so this is being put back in temporarily while the
browser plugin is modified to remove this function.

Some games don't catch GL errors via glGetError, so there's a
possibility that an error will pass through to the capture calls,
causing a false failure.

The most simple solution is to just clear the error flag on each capture
call.

With certain devices (AVerMedia C985 and LGP), audio timestamps are
bad, and a 50ms threshold of audio data "smoothing" (making consecutive
audio packets seamless with one another) isn't enough to handle bad
consecutive timestamp values.  After testing, 70ms sufficiently solves
the issue.

This will probably go away once proper file name formatting is
implemented (https://obsproject.com/mantis/view.php?id=361)

Given a config file with some value "foo=\\n" querying the value of foo
would return {'\\', '\n', 0}, instead of {'\\', 'n', 0}