Skia rgb32 not working

[djowel]

Related to https://github.com/cycfi/artist/commit/5ed608524444a0bfe784f74b01ea2191d9cd4be7

Pinging bnert

I'm getting weird results when the pixel format is rgb32:

https://github.com/cycfi/artist/blob/5ae04694544ec3677a8d67ca1e51153e86882cd9/lib/include/artist/image.hpp#L34

I'll remove rgb32 for now. rgba32 is most important and it's working just fine. I'd appreciate it if you can take a look.

I haven't tested the other options gray8 and rgb16. It's a good idea to have these tested as well.