Allows the ability for one encoder to defer to another in case of
failure or unsupported feature.  Okay, fine, it's mostly a hack so the
new NVENC encoder can fall back to the FFmpeg encoder if NV12 textures
aren't in use, that way it does not have to implement raw fallback
support itself.  The settings and properties are pretty much the same,
so there's no reason not to utilize it in order to save time that could
otherwise be spent more productively.

(This commit also modifies UI)

Adds a universal function for determining whether video output is
currently active, rather than having to use video_output_active() on the
value returned by obs_get_video().

obs_set_cmdline_args copies the arguments passed to it. This means that
the array passed will never require it to be modified.

Closes obsproject/obs-studio#1507

Useful when you need to modify transform properties of group sub-items
and have the transform available immediately (group sub-items always
automatically defer their transform update to the next frame).

Allows the ability to set custom global OBS context private data for
allowing the frontend to communicate variables with plugins in an
indirect way.

These function declarations were in the wrong section.

Rather than relying on the encoder's type identifier, additionally allow
the ability to get defaults via an encoder object.

This paves the way for plugins to have access to the argc/argv used
when spawning OBS. Notably, this will allow a patch to obs-browser that
passes through command line arguments on macOS to CEF.

(This commit also modifies UI)

Removes obs_scene::group_sceneitem and replaces it with
obs_scene::is_group.  Changes a number of other functions related to
groups so that a group is not inherently tied to a specific scene, and
helps allow a single group to be referenced in multiple scenes if
desired.

Allows the ability to group scene items.  Groups internally are
sub-scenes, which allows the ability to add unique filters and
transforms to each group.

Closes obsproject/obs-studio#1300

Closes obsproject/obs-studio#1263

Adds obs_add_raw_video_callback() and obs_remove_raw_video_callback()
functions which allow the ability to get raw video frames without
necessarily needing to create an output.

Allows the ability to get the last rendered texture of the main output
view.

Closes jp9000/obs-studio#1203

Prevents SWIG from being able to process certain declarations that would
cause binding generation errors/warnings.

Allows the ability to have a callback invoked every time video ticks.
Particularly useful for scripting.

(Note: This commit also modifies UI and test)

This makes it so that main preview panes are rendered with the main
output texture rather than re-rendering the main view.  The view will
render all objects again, whereas the output texture will be a single
texture render of the same exact thing.

Also fixes some abnormal artifacting when scaling the main preview pane.

Decoupling the audio from the video causes the audio to be played right
when it's received rather than attempt to sync up to the video frames.
This is useful with certain async sources/devices when the audio/video
timestamps are not reliable.

Naturally because it plays audio right when it's received, this should
only be used when the async source is operating in unbuffered mode,
otherwise the video frame timing will be out of sync by the amount of
buffering the video currently has.

Allows setting and sharing of user data for sources and scene items.

Some plugins are using the OBS_VERSION macro obs-config.h to specify the
current version, which will bake the string in to the plugin, making it
so that if the plugin is not replaced for a patch, it could potentially
have the incorrect version.  This makes it so that a plugin/frontend can
get the current version string that's baked in to libobs itself.

This allows the ability for certain types of modules (particularly
scripting-related modules) to initialize extra data when all other
modules have loaded.  Because front-ends may wish to have custom
handling for loading modules, the front-end must manually call
obs_post_load_modules after it has completed loading all plug-in
modules.

Closes jp9000/obs-studio#965

For specific types of transitions (stingers in this case), there is no
blending done of the two targets, so wasting GPU resources rendering
them to textures is unnecessary.

Allows the ability to change the output type in case one service
requires a different output type.

NOTE: This should be considered a temporarily yet simple solution to a
specific problem: support for RTMP-like outputs.  This will allows
seamless integration of supporting different RTMP-like output types
within the same service.  This should probably be replaced with a more
ideal solution later, such as implementing a completely different
service type instead, when time permits.

(This commit also modifies obs-outputs)

Adds a lock checkbox to allow the user to lock a specific scene item.

Closes jp9000/obs-studio#949

Allows directly rendering a non-async source without going through
filters.