app: fix jumping around of newly created images

Put the center_image_on_size_allocate() code into the canvas'
size-allocate callbacck.

As a side effect we now have a flag in GimpDisplayShell which
indicates that there will be a size allocate before the next frame, so
simply skip drawing the canvas completely. This fixes new images
jumping around when they are first shown.

(cherry picked from commit c0480f502d)

(this fix is actually a side effect from fixing something else in
master)

app/display/gimpdisplayshell.h

@@ -179,6 +179,7 @@ struct _GimpDisplayShell
   gboolean           zoom_on_resize;
 
   gboolean           size_allocate_from_configure_event;
+  gboolean           size_allocate_center_image;
 
   /*  the state of gimp_display_shell_tool_events()  */
   gboolean           pointer_grabbed;