Optional experimental WebGL color conversion and drawing

Shader nicked from Broadway.js -- https://github.com/mbebenita/Broadway/blob/master/Player/canvas.js

Definitely outperforms our JS conversion on full HD movies in Firefox, Chrome :D

Works on Safari desktop if WebGL is manually enabled. Not available on iOS. :(

On IE this requires expanding luminance textures to RGBA, which seems slow.
Additionally, WebGL is unreliably slow under Parallels, so undertested on IE right now.