Add a layout debug option

This will be used to show box model borders.

docs/reference/gtk/running.sgml

@@ -218,7 +218,10 @@ additional environment variables.
       <term>resize</term>
       <listitem><para>Highlight resizing widgets</para></listitem>
     </varlistentry>
-
+    <varlistentry>
+      <term>layout</term>
+      <listitem><para>Show layout borders</para></listitem>
+    </varlistentry>
   </variablelist>
   The special value <literal>all</literal> can be used to turn on all
   debug options. The special value <literal>help</literal> can be used

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 */