usp10: Fix next index for RTL MultipleSubst.

MultipleSubst will expand the glyph array to the right, so when the
writing direction is RTL the next index does not need to be modified by
the new count of glyphs.

Signed-off-by: Aric Stewart <aric@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/usp10/opentype.c

@@ -940,7 +940,10 @@ static INT GSUB_apply_MultipleSubst(const OT_LookupTable *look, WORD *glyphs, IN
                 TRACE("\n");
             }
 
-            return glyph_index + (sub_count * write_dir);
+            if (write_dir > 0)
+                return glyph_index + sub_count;
+            else
+                return glyph_index - 1;
         }
     }
     return GSUB_E_NOGLYPH;