[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.

src/sfnt/sfobjs.c

@@ -1107,13 +1107,7 @@
       /* Don't bother to load the tables unless somebody asks for them. */
       /* No need to do work which will (probably) not be used.          */
       if ( face->variation_support & TT_FACE_FLAG_VAR_FVAR )
-      {
-        if ( tt_face_lookup_table( face, TTAG_glyf ) != 0 &&
-             tt_face_lookup_table( face, TTAG_gvar ) != 0 )
-          flags |= FT_FACE_FLAG_MULTIPLE_MASTERS;
-        if ( tt_face_lookup_table( face, TTAG_CFF2 ) != 0 )
-          flags |= FT_FACE_FLAG_MULTIPLE_MASTERS;
-      }
+        flags |= FT_FACE_FLAG_MULTIPLE_MASTERS;
 #endif
 
       root->face_flags = flags;

src/truetype/ttgxvar.c

@@ -2217,20 +2217,6 @@
     {
       FT_TRACE2(( "FVAR " ));
 
-      /* both `fvar' and `gvar' must be present */
-      if ( FT_SET_ERROR( face->goto_table( face, TTAG_gvar,
-                                           stream, &table_len ) ) )
-      {
-        /* CFF2 is an alternate to gvar here */
-        if ( FT_SET_ERROR( face->goto_table( face, TTAG_CFF2,
-                                             stream, &table_len ) ) )
-        {
-          FT_TRACE1(( "\n" ));
-          FT_TRACE1(( "TT_Get_MM_Var: `gvar' or `CFF2' table is missing\n" ));
-          goto Exit;
-        }
-      }
-
       if ( FT_SET_ERROR( face->goto_table( face, TTAG_fvar,
                                            stream, &table_len ) ) )
       {
@@ -2673,8 +2659,16 @@
     FT_TRACE5(( "\n" ));
 
     if ( !face->is_cff2 && !blend->glyphoffsets )
-      if ( FT_SET_ERROR( ft_var_load_gvar( face ) ) )
+    {
+      /* While a missing 'gvar' table is acceptable, for example for */
+      /* fonts that only vary metrics information or 'COLR' v1       */
+      /* `PaintVar*` tables, an incorrect SFNT table offset or size  */
+      /* for 'gvar', or an inconsistent 'gvar' table is not.         */
+      error = ft_var_load_gvar( face );
+      if ( error != FT_Err_Table_Missing && error != FT_Err_Ok )
         goto Exit;
+      error = FT_Err_Ok;
+    }
 
     if ( !blend->coords )
     {