[sfnt] Don't require 'gvar' table to support variation fonts.

Fixes #1172.

* src/sfnt/sfobjs.c (sfnt_load_face): Tag font as Multiple Masters font if
`fvar` is present; do not require other tables to be present.

* src/truetype/ttgxvar.c (tt_set_mm_blend): Allow for a missing 'gvar' table
when setting variation coordinates.  However, if a 'gvar' table is actually
present, do perform a sanity check and fail on malformedness.
(TT_Get_MM_Var): Don't assume 'fvar' needs 'gvar' or 'CFF2 tables in all
cases, which is an overly tight check.
2 files changed