gdkwindow: avoid updating background pattern if it matches previous

Background patterns are often updated when style changes. In many cases,
the new pattern will match the previous. We can optimize out the
invalidation that will occur upon resetting the same pattern.

gdk/gdkwindow.c

@@ -5977,10 +5977,25 @@ gdk_window_set_background_rgba (GdkWindow     *window,
                                 const GdkRGBA *rgba)
 {
   cairo_pattern_t *pattern;
+  GdkRGBA prev_rgba;
 
   g_return_if_fail (GDK_IS_WINDOW (window));
   g_return_if_fail (rgba != NULL);
 
+  /*
+   * If the new RGBA matches the previous pattern, ignore the change so that
+   * we do not invalidate the window contents.
+   */
+  if ((window->background != NULL) &&
+      (cairo_pattern_get_type (window->background) == CAIRO_PATTERN_TYPE_SOLID) &&
+      (cairo_pattern_get_rgba (window->background,
+                               &prev_rgba.red,
+                               &prev_rgba.green,
+                               &prev_rgba.blue,
+                               &prev_rgba.alpha) == CAIRO_STATUS_SUCCESS) &&
+      gdk_rgba_equal (&prev_rgba, rgba))
+    return;
+
   pattern = cairo_pattern_create_rgba (rgba->red, rgba->green,
                                        rgba->blue, rgba->alpha);