webp: Decode animations frame by frame

[sophie-h]

Previously, animated WebPs could take some time
until they started playing and would consume more memory.

[sophie-h]

We can also drop the "skip invalid frames" part. But since the iterator is silently ending with errors I thought we might as well skip invalid frames.

[fintelia]

This code will change a bunch when we switch to image-webp. However, that'll have to wait for a major release (the crate relies on being able to Seek the underlying reader). So in the meantime I think this change does make sense.

Relatedly, if you have thoughts about API changes we should make for animation decoding, please do share them!

[fintelia]

I don't have strong opinions on how invalid frames should be handled, but one thing to be aware of is that frames can be composited onto the image canvas rather than replacing it. This means that portions of the animation may be left unchanged between frames, and thus skipping a single frame can potentially corrupt the displayed contents for all subsequent frames.

[sophie-h]

I don't have strong opinions on how invalid frames should be handled

I dropped it from this MR. I don't know what the reference implementation does.