This removes the trick of drawing to a 2D canvas. It was corrupting the
alpha in RGBM images, which caused subtle banding in the Suzanne
reflection. Moreover it added complexity to the demo code.
This commit will increase the pre-zipped WASM size by 17k, but I think
it's worth it.
This also removes one of the cmgen passes in the WebGL build; we were
using PNG instead of RGBM for the skybox to work around the banding
issue that this fixes.
7b36ed4638
