Fix some warnings on VS2013 /W4

Minor fixes/disables to compile warning free on VS2013 at /W4.
I build nanovg as .cpp and I don't get C4127 at all with that unused macro. For other stuff where I get C4127 I use this, but it's C++ template:
https://github.com/bkaradzic/bx/blob/master/include/bx/bx.h#L22