[truetype] Refactor compensation color. * src/truetype/ttinterp.h (TT_Round_Func): Change the last argument. * src/truetype/ttinterp.c (Ins_ROUND, Ins_NROUND, Ins_MDAP, Ins_MIAP, Ins_MDRP, Ins_MIRP): Move compensation retrieval from here... (Round_*): ... to here. * src/truetype/ttobjs.c (tt_size_init_bytecode): Reserve zero compensation at color index 3.