[truetype] Fix style name handling for variation fonts.

* include/freetype/internal/tttypes.h (TT_FaceRec): New field
`non_var_style_name`.

* src/sfnt/sfobjs.c (sfnt_load_face): Initialize `non_var_style_name`.
(sfnt_done_face): Free `non_var_style_name`.

* src/truetype/ttgxvar.c (TT_Set_Named_Instance): Restore non-VF style name
if switching back to non-VF mode.
3 files changed