decode_gif: add transparency support and image offsets #8419
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This PR now properly handles transparency within the GIF, and also allows each individual image to start at its own Top and Left offset. For docs on how transparency is encoded in GIFs, see https://giflib.sourceforge.net/whatsinagif/animation_and_transparency.html
Turns out the decoder was unable to properly decode https://en.wikipedia.org/wiki/GIF#/media/File:Rotating_earth_(large).gif, but now it's fine.
Benchmark. Slighly slower than before but the relative speed compared with PIL is similar to before.