fons__tmpalloc alignment

I got a SIGBUS (7) inside one of the stb_truetype-functions when running NanoVG on Android (Galaxy S2). Tracked it down to fons__tmpalloc not returning sufficiently aligned pointers. 

The attached fix works fine for me, but I think `uintptr_t` might be C99 only? Compiles fine with the Android NDK (and "-std=c99"), as well as Visual C++ 2013 ("Compile as C Code (/TC)").