Simplified set_clip_mask. Bitmap can not be 0 according to doc and type.

Rev: src/post_modules/GTK2/source/gdkgc.pre:1.4

src/post_modules/GTK2/source/gdkgc.pre

@@ -135,13 +135,9 @@ void set_clip_origin(int x, int y)
 void set_clip_mask(GDK2.Bitmap mask)
 //! Set the clip mask to the specified GDK2.Bitmap
 {
-  if (args==1) {
   struct object *o;
   get_all_args("set_clip_mask",args,"%o",&o);
   gdk_gc_set_clip_mask(GDK_GC(THIS->obj),GDK_DRAWABLE(get_gobject(o)));
-  } else
-    gdk_gc_set_clip_mask(GDK_GC(THIS->obj),0);
-
   RETURN_THIS();
 }