Due to the way JPEG XL encodes its lossless images, it is sometimes
interesting to embed a large image and crop the result at the end. This
patch adds the functionality to crop a frame.
Note that JPEG XL supports image composition (almost like layers in
image editing software programs) and I tried to make these changes be
a step toward image composing. It's a small step as we are still unable
to read multiple frames, and we only support the `kReplace` blending
mode.
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