Independent X and Y scaling

[technosaurus]

I submitted a pull request #68 that (passably?) uses a scalex and scaley to scale and stretch/compress images to a different aspect ration. I am trying to add an option to replace cairo/rsvg in jwm with nanosvg (requires independent x-y scaling) as well as demonstrate xcb with a basic image viewer

I think this will be useful for other purposes, but I wasn't sure if nanosvgrast had declared a stable API, so I reimplemented all the functions with a single scale parameter to call the new *Full versions.