[CoreText] Don't leak the CTLine object (and everything that hangs off it)

Patch from Jonathan Kew.

src/hb-coretext.cc

@@ -348,5 +348,7 @@ _hb_coretext_shape (hb_shape_plan_t    *shape_plan,
     }
   }
 
+  CFRelease (line);
+
   return true;
 }