app/display: let cairo render the checkerboard

Instead of blending the scaled image data onto the checkerboard and
then painting this image to the screen, render the image data into
an ARGB cairo image surface. Then paint a checkerboard on the canvas
and the image on top of it.

app/display/gimpdisplayshell-handlers.c

@@ -592,6 +592,12 @@ gimp_display_shell_check_notify_handler (GObject          *config,
   GimpCanvasPaddingMode padding_mode;
   GimpRGB               padding_color;
 
+  if (shell->checkerboard)
+    {
+      cairo_pattern_destroy (shell->checkerboard);
+      shell->checkerboard = NULL;
+    }
+
   gimp_display_shell_get_padding (shell, &padding_mode, &padding_color);
 
   switch (padding_mode)