Use WebGL for rendering where available

Still falls back to 2d canvas for old browsers.  In newer browsers supporting WebGL though, there is a good performance gain.  According to a profile, `YCbCrToRGBA` is currently the function where the most time is spent.  By moving the RGB conversion to the GPU with a WebGL shader, I see a 5-10% CPU usage decrease in Chrome and Firefox, and about 15-20% decrease in Safari with WebGL enabled.  You can try it for yourself here http://devongovett.github.io/jsmpeg/test.html though the loading indicator doesn't work with WebGL so just wait for it to load.