Added code to fallback to Yosemite or pre-Yosemite system font in case current system font can't be found or is in an unusable format. San Francisco font used on current OS X versions is in a .otf file whih presumably means it has CFF outlines that stb_truetype can't use.

#ifndef PNG_H_
#define PNG_H_

void save_png24(FILE *f, uint32_t *buffer, uint32_t width, uint32_t height, uint32_t pitch);
void save_png(FILE *f, uint32_t *buffer, uint32_t width, uint32_t height, uint32_t pitch);

#endif //PNG_H_