[subset] Match hb rounding to fonttools for avg char width.

Update Muli and Roboto input test fonts to recalc class values. FontTools recalcs class values during instancing while harfbuzz does not (like the ft subsetter).

src/hb-ot-os2-table.hh

@@ -223,7 +223,7 @@ struct OS2
       }
     }
 
-    return num ? (unsigned) roundf (total_width / num) : 0;
+    return num ? (unsigned) roundf ((double) total_width / (double) num) : 0;
   }
 
   bool subset (hb_subset_context_t *c) const