Support for VP8/VP9 alpha transparency

[zavitax]

I wonder if it's possible to play back WEBM videos with alpha channel preserving transparency with ogv.js
At the moment it forces a black background on the playback screen.

[bvibber]

Probably could be done, but would require some additions to yuv-canvas to support carrying and displaying the alpha channel. I won't rule it out, especially if it turns out to be easy to do, but be aware it's not on my first tier of cleanup work to do. :)