Auto merge of #15122 - anholt:webgl-premultiply, r=emilio

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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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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Cargo.lock

@@ -2273,6 +2273,7 @@ dependencies = [
  "audio-video-metadata 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "bitflags 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bluetooth_traits 0.0.1",
+ "byteorder 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "canvas_traits 0.0.1",
  "caseless 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "cmake 0.1.20 (registry+https://github.com/rust-lang/crates.io-index)",