This intentionally will not build without --enable-ffmpeg for now.
We add new functions to the RenderAPI called AVI_Think and AVI_SetParm. Game
developer is supposed to call AVI_Think each frame and set video playback
parameters through AVI_SetParm function.
What's tested:
* Intro cinematics.
* Looping videos.
* Playing back an hour long video.
* API extension (PrimeXT's avi branch at https://github.com/a1batross/PrimeXT).
What's broken:
* There is no way to seek a video file, only rewind to the start.
