Backend: rawdraw

I just came across https://github.com/cntools/rawdraw and thought it might be worth trying as a highly-performant **multiplatform** (Windows, Linux, macOS, Android, ...) no-s**t backend.

So if anyone would be looking for something very light but still heavily multiplatform with zero dependencies, this might be the way to go. We will then happily merge this backend into this repository :wink:.

Related: https://github.com/Immediate-Mode-UI/Nuklear/issues/464

Start by reviewing the proposed rawdraw backend at https://github.com/cntools/rawdraw and the related Nuklear issue #464. The issue names no repository files, entry points, or tests; completion would require agreeing on the integration scope and successfully merging a multiplatform, zero-dependency backend.

Written by the indexing model from the issue text.