Using separate shim to include nanovg GL glue code

I've noticed that you make nanovg-rs depend on your fork of nanovg which includes nanovg GL implementation glue code. However, this requires constant upgrading of the fork because there may be bug fixes there (in the current forked version, for example, `nvgCreateImageMem` fails with a segfault; in the original master it is fixed).

I've implemented a small shim library which depends on nanovg and includes its headers with appropriate flags to generate nanovg GL code generation. It is also built with premake so it should be cross-platform. Now we can use upstream nanovg for building.