LevelBar: Avoid unnecessarily copying allocation

Just use the original allocation, instead of copying it and passing that

gtk/gtklevelbar.c

@@ -462,9 +462,8 @@ gtk_level_bar_allocate_trough_continuous (GtkLevelBar *self,
   inverted = gtk_level_bar_get_real_inverted (self);
 
   /* allocate the empty (unfilled) part */
-  block_area = *allocation;
   gtk_css_gadget_allocate (self->priv->block_gadget[inverted ? 0 : 1],
-                           &block_area,
+                           allocation,
                            baseline,
                            out_clip);