[gimp] app: add comment about cairo_t and double buffering

[Michael Natterer <mitch src gnome org>]

commit c0e475e7e2e0b2279b054f7c2aaa5324df469154
Author: Michael Natterer <mitch gimp org>
Date:   Sat Aug 28 11:44:11 2010 +0200

    app: add comment about cairo_t and double buffering

 app/display/gimpdisplayshell-callbacks.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/app/display/gimpdisplayshell-callbacks.c b/app/display/gimpdisplayshell-callbacks.c
index b2181c3..483c465 100644
--- a/app/display/gimpdisplayshell-callbacks.c
+++ b/app/display/gimpdisplayshell-callbacks.c
@@ -404,6 +404,9 @@ gimp_display_shell_canvas_expose (GtkWidget        *widget,
             gdk_window_begin_paint_region (eevent->window, eevent->region);
         }
 
+      /*  create the cairo_t after enabling double buffering, or we
+       *  will get the wrong window destination surface
+       */
       cr = gdk_cairo_create (gtk_widget_get_window (shell->canvas));
       gdk_cairo_region (cr, eevent->region);
       cairo_clip (cr);