Add WebP roundtrip fuzzing target

[Shnatsel]

This will detect issues such as #2171, as well as instances of incorrect encoding or decoding.

It is currently limited to RGBA, since I didn't find an easy way to use RGB format conditionally. The decoding always returns RGBA, which makes comparisons of the data a little tricky.

[fintelia]

Looks good to me.

Though I'll note that this actually wouldn't have caught the linked issue. That was caused by a bug in the encoder that caused it to write slightly corrupt images that were missing a trailing padding byte at the end of the file. The reason it slipped through was that out decoder isn't that disciplined about ensuring that extra padding bytes are present if there's enough input data to decode all the pixels