container: Prepare to prune render nodes

Add a should_propagate function for render nodes. Eventually,
this is meant to avoid creating render nodes for child widgets
that are outside the parents clip area. Since we don't have
that information available right now, just filter out nondrawable
children for now.

gtk/gtkcontainer.c

@@ -3405,6 +3405,16 @@ typedef struct {
   GArray *child_infos;
 } RenderData;
 
+static gboolean
+should_propagate_node (GtkWidget  *child,
+                       RenderData *data)
+{
+  if (!_gtk_widget_is_drawable (child))
+    return FALSE;
+
+  return TRUE;
+}
+
 static void
 propagate_render_node (GtkWidget *child,
                        gpointer   data_)
@@ -3473,6 +3483,9 @@ collect_child_infos (GtkWidget *widget,
   GList *siblings;
   GdkWindow *window;
 
+  if (!should_propagate_node (widget, data))
+    return;
+
   info.child = widget;
   info.window_depth = G_MAXINT;
 
@@ -3489,7 +3502,6 @@ collect_child_infos (GtkWidget *widget,
   g_array_append_val (data->child_infos, info);
 }
 
-
 void
 gtk_container_propagate_render_node (GtkContainer  *container,
                                      GskRenderer   *renderer,
@@ -3501,6 +3513,7 @@ gtk_container_propagate_render_node (GtkContainer  *container,
   data.container = container;
   data.renderer = renderer;
   data.parent = parent_node;
+
   data.child_infos = g_array_new (FALSE, TRUE, sizeof (ChildOrderInfo));
 
   gtk_container_forall (container, collect_child_infos, &data);