Fix some warnings on VS2013 /W4

Minor fixes/disables to compile warning free on VS2013 at /W4.
I get

src/nanovg.c(701) : warning C4100: 'ctx' : unreferenced formal parameter
src/nanovg.c(740) : warning C4100: 'ctx' : unreferenced formal parameter
src/nanovg.c(767) : warning C4100: 'ctx' : unreferenced formal parameter
src/nanovg.c(793) : warning C4100: 'ctx' : unreferenced formal parameter
src/nanovg.c(1231) : warning C4100: 'fringe' : unreferenced formal parameter
src/nanovg.c(1291) : warning C4100: 'fringe' : unreferenced formal parameter

with the original version in vs2013. I was totally confused for a while, but it's actually whether it's in a .c or .cc! :(

i.e. 
---
#pragma warning(disable: 4127)  // conditional expression is constant
#define NVG_NOTUSED(v) do { (void)(1 ? (void)0 : ( (void)(v) ) ); } while(0)

void f(int a) {
  NVG_NOTUSED(a);
}
---

is fine in a.cpp at /W4, but emits 
a.c(4) : warning C4100: 'a' : unreferenced formal parameter
when in a.c.

Maybe @bkaradzic has some more magic fixes per issue #78? Or just give in and #pragma warning(disable: 4100) in the .c files I guess?