Implement some basic audio mixing
Implement auto output resampling (if requested)

If there are for example more than one audio outputs and they have
different sample rates or channels and such, this will allow automatic
conversion of that audio to the request formats/channels/rates (but only
if requested).
Add Libavutil dependency for obs-ffmpeg plugin

av_frame_alloc, av_frame_free, among others, live in libavutil
Implement automatic video scaling (if requested)

Add a scaler interface (defaults to swscale), and if a separate output
wants to use a different scale or format than the default output format,
allow a scaler instance to be created automatically for that output,
which will then receive the new scaled output.
Use forward instead of include

Had a bit of an include loop going on there which caused this issue to
occur.
Fix improper structure dependency

I actually did compile that last commit and misread the failed projects
as 0.  I'm just going to put the conversion stuff in video-io.h stuff
because it requires it anyway, and video-scaler.h already depends on
video-io.h for the video_format enum anyway.