GtkGLArea: Add resize signal

This is very useful, as almost all GL code wants to recalculate
the cameras each time the window size changes.

gtk/gtkglarea.h

@@ -68,6 +68,9 @@ struct _GtkGLAreaClass
   /*< public >*/
   gboolean       (* render)         (GtkGLArea        *area,
                                      GdkGLContext     *context);
+  void           (* resize)         (GtkGLArea        *area,
+				     int               width,
+				     int               height);
 
   /*< private >*/
   gpointer _padding[6];