We're still not calculating alpha blending correctly. In the attached image:
left is WebGL, right is Canvas. Notice the dark line when blending from
transparent to white.
Due to stability issues, specifying premultiplied alpha at the webgl attribute
level isn't an option. I think the best route is to do this ourselves in code
(e.g., gl.blendFunc(ONE, INVERSE_ALPHA) and premultiply the color channels when
loading the image).