WebM Movie Player (VP9/Opus)

Now, this is what I call a collaboration.
This, finally, implements our movie player. It can play webm containers that include vp9 video and opus audio. As a bonus, it supports multiple tracks and tries to select the track that matches the game language, if possible.
potential optimization: move the allocation step out of the loop and only (re-)allocate if the existing buffer is not big enough