[style] Fall back to GPOS 'size' feature for optical size in style API

src/hb-style.cc

@@ -112,9 +112,11 @@ hb_style_get_value (hb_font_t *font, hb_tag_t tag)
     return face->table.OS2->is_italic () || face->table.head->is_italic () ? 1 : 0;
   case HB_STYLE_TAG_OPTICAL_SIZE:
   {
-    unsigned int lower, upper;
+    unsigned int lower, design, upper;
     return face->table.OS2->v5 ().get_optical_size (&lower, &upper)
 	   ? (float) (lower + upper) / 2.f
+	   : hb_ot_layout_get_size_params (face, &design, nullptr, nullptr, nullptr, nullptr)
+	   ? design / 10.
 	   : 12.f;
   }
   case HB_STYLE_TAG_SLANT: