[truetype] Refine some GX sanity tests. Use the `gvar' table size instead of the remaining bytes in the stream. * src/truetype/ttgxvar.h (GX_BlendRec): New field `gvar_size'. * src/truetype/ttgxvar.c (ft_var_load_gvar): Set `gvar_size'. (ft_var_readpackedpoints, ft_var_readpackeddeltas: New argument `size'. (tt_face_vary_cvt, TT_Vary_Apply_Glyph_Deltas): Updated.