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 FONT_H_
#define FONT_H_

uint8_t *default_font(uint32_t *size_out);

#endif //FONT_H_