mint-arena: (re)move defines at top of q_shared.h

Move three non-engine defines at the top of q_shared.h to bg_public.h.
Remove the engine defines.
mint-arena: Move fontInfo_t to tr_types.h
mint-arena: Add %x to bg_lib.c's sscanf from Tremulous
mint-arena: Add warning for unsupported format chars in sscanf