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.
Should this ifdef be one line higher? Currently you'd end up with the following if VIDEO_AVAILABLE were not defined:

```cpp
class VPX
{
    VPX() { }

public:
}
```