gdk: Do final copy with OPERATOR_SOURCE in end_implicit_paint

For windows with alpha channel, the previous contents would otherwise
not be erased. Visible for example in the status icon code.

Thanks to Thomas Wood for noticing.

gdk/gdkwindow.c

@@ -2833,6 +2833,7 @@ gdk_window_end_implicit_paint (GdkWindow *window)
       cr = gdk_cairo_create (private->impl);
       gdk_cairo_set_source_pixmap (cr, paint->pixmap,
                                    paint->x_offset, paint->y_offset);
+      cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
       gdk_cairo_region (cr, paint->region);
       cairo_fill (cr);
       cairo_destroy (cr);