Change graphics library to Skia

[jmidt]

Right now, we are using NanoVG, which is not actively maintained. We would like to change this to a more modern alternative, such as Skia.

All calls to NanoVG is wrapped in the small wrapper NanoCanvas, so it should be simple to change out the backend without having to rewrite any graphics code in OTTO. We can then extend the wrapper to expose the more advanced features of Skia.