WebP and WebM support

[IonoclastBrigham]

Looks like there's prior art out there for implementing these libraries for racket, but I can't be certain as to their quality or relevance. Hacking minimal wrappers for one or both formats may be easier than trying to wrangle someone else's code.

[lehitoskin]

The most basic functionality I need from those are the frames. I don't need full support of the webp or webm standards so long as I can disassemble their chunks and grab the frames as bitmaps (or ARGB bytes).