disable GL debug checks by default

Checking for GL errors stops the GPU pipeline and can
dramatically slow down the rendering on some browsers.
Huge difference in Chrome (<1ms/frame vs 5-6ms/frame),
smaller difference in IE.
Fix output cropping in WebGL drawing

https://github.com/brion/ogv.js/issues/92

Currently assumes that luma and chroma textures are all the same size in output pixels.
Merge branch 'master' of github.com:brion/ogv.js