[sfnt] Handle variable COLR v1 fonts without delta set index map

Fixes #1178.

* src/sfnt/ttcolr.c (get_deltas_for_var_index_base): Set outer index to
0 and inner index to the delta index when retrieving deltas if the
`COLR` table has no delta set index map.

src/sfnt/ttcolr.c

@@ -604,9 +604,8 @@
       }
       else
       {
-        /* TODO: Direct lookup case not implemented or tested yet. */
-        FT_ASSERT( 0 );
-        return 0;
+        outer_index = 0;
+        inner_index = loop_var_index;
       }
 
       deltas[i] = mm->get_item_delta( FT_FACE( face ), &colr->var_store,