Add a layout debug option

This will be used to show box model borders.

gtk/gtkdebug.h

@@ -55,7 +55,8 @@ typedef enum {
   GTK_DEBUG_INTERACTIVE     = 1 << 17,
   GTK_DEBUG_TOUCHSCREEN     = 1 << 18,
   GTK_DEBUG_ACTIONS         = 1 << 19,
-  GTK_DEBUG_RESIZE          = 1 << 20
+  GTK_DEBUG_RESIZE          = 1 << 20,
+  GTK_DEBUG_LAYOUT          = 1 << 21
 } GtkDebugFlag;
 
 #ifdef G_ENABLE_DEBUG

gtk/gtkmain.c

@@ -181,7 +181,8 @@ static const GDebugKey gtk_debug_keys[] = {
   {"interactive", GTK_DEBUG_INTERACTIVE},
   {"touchscreen", GTK_DEBUG_TOUCHSCREEN},
   {"actions", GTK_DEBUG_ACTIONS},
-  {"resize", GTK_DEBUG_RESIZE}
+  {"resize", GTK_DEBUG_RESIZE},
+  {"layout", GTK_DEBUG_LAYOUT}
 };
 #endif /* G_ENABLE_DEBUG */