[sfnt, truetype] Don't require gvar for font to support variations

Fixes #1172 (closed).

• src/sfnt/sfobjs.c (sfnt_load_face): If fvar is present, tag font with multiple masters tag also if gvar and CFF2 are missing, but COLR is present.
• src/truetype/ttgxvar.c (tt_set_mm_blend): Allow for a missing gvar table when setting variation coordinates but if a gvar table is present still sanity check it and fail on malformedness. (TT_Get_MM_Var): Don't assume fvar needs gvar or CFF2 in all cases, which is an overly tight check.