allows for nanovg_gl prototypes to be declared simultaneously

This is a simple fix to enable `#incldue <nanovg_gl.h>`to define prototypes for the various backends simultaneously.

This is an absolute requirement for my code.  Previously, I included `nanovg_gl.h` multiple times with an `#undef NANOVG_GL3_H` (or `#undef NANOVG_GL_H`) after each instance.  That workaround is no longer possible since then, e.g., `NVGLtextureflags`  would now be declared multiple times.  The present solution is much more elegant anyway.