vurtun
Non OpenGL SDL2 backend that uses SDL2_gfx
It's based on the old 1.2 backend as you can see in my commits. It doesn't clash with the other pull request/issue currently open (which is sdl_native) and the old SDL1.2 folder was sdl1.2_gfx not sdl_gfx so I think that's sufficient.
I could rename the folder sdl2_gfx or similar but that would be inconsistent/confusing since the opengl ones are sdl_openglX not sdl2_openglX even though they've always used SDl2.
Also I can create a new branch with a single commit if you prefer but I wanted to show the 1.2 to 2 porting process if people were interested.
Lastly, I also found an unrelated bug. If you define VARARGS the compile fails unless the user includes stdarg.h before including nuklear. This is because va_list is undefined but needed even in the header portion of nuklear.h unlike for STANDARD_IO etc. where they're only needed in the implementation.
I can fix that in a separate pull request or add it to this one. I think it's just adding stdarg.h here
