[cff2] Use a shrink instead of resize

Such that we can free the allocation.

src/hb-cff2-interp-cs.hh

@@ -56,7 +56,7 @@ struct blend_arg_t : number_t
   void reset_blends ()
   {
     numValues = valueIndex = 0;
-    deltas.resize (0);
+    deltas.shrink (0);
   }
 
   unsigned int numValues;