webgl: premultiplication, y flipping, and format conversion
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This series implements a bunch of the texture unpack path features for WebGL. There are known issues with big-endian systems noted in the comments, but I don't know of a clean way to cast from `Vec<u16>` to `Vec<u8>` (which, if we had one, would make this code much cleaner anyway).
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536c0d7499
