container: Implement get_opaque_rect()

We find the first child that covers >80% of the container and return
that.

This is a nice speedup for the common case of a GtkWindow being covered
by a large opaque background.
It will fall apart for fancy themes that play with transparency or for
small windows because the shadow region gets too large.

But then we just scan the whole node tree.

We could think about adapting the 80% number, because that wasn't chosen
with any real scientific data behind it.

gsk/gskrendernodeimpl.c

@@ -3309,6 +3309,51 @@ gsk_container_node_diff (GskRenderNode *node1,
   gsk_render_node_diff_impossible (node1, node2, data);
 }
 
+static gboolean
+gsk_container_node_get_opaque_rect (GskRenderNode   *node,
+                                    graphene_rect_t *opaque)
+{
+  GskContainerNode *self = (GskContainerNode *) node;
+  graphene_rect_t child_opaque;
+  double size, child_size, desired_size;
+  guint i;
+
+  for (i = 0; i < self->n_children; i++)
+    {
+      if (gsk_render_node_get_opaque_rect (self->children[i], opaque))
+        break;
+    }
+
+  if (i == self->n_children)
+    return FALSE;
+
+  size = opaque->size.width * opaque->size.height;
+  /* the 80% is random. I just want it to be low enough to catch
+   * rounded corners, but not so small it catches on to .view
+   * backgrounds for the smaller child of a paned.
+   */
+  desired_size = node->bounds.size.width * node->bounds.size.height * 0.8; 
+
+  for (i++; i < self->n_children; i++)
+    {
+      if (size >= desired_size)
+        break;
+
+      if (!gsk_render_node_get_opaque_rect (self->children[i], &child_opaque))
+        continue;
+
+      child_size = child_opaque.size.width * child_opaque.size.height;
+      /* We allow == here because we want to find the topmost opaque child */
+      if (child_size < size)
+        continue;
+
+      *opaque = child_opaque;
+      size = child_size;
+    }
+
+  return TRUE;
+}
+
 static void
 gsk_container_node_class_init (gpointer g_class,
                                gpointer class_data)
@@ -3320,6 +3365,7 @@ gsk_container_node_class_init (gpointer g_class,
   node_class->finalize = gsk_container_node_finalize;
   node_class->draw = gsk_container_node_draw;
   node_class->diff = gsk_container_node_diff;
+  node_class->get_opaque_rect = gsk_container_node_get_opaque_rect;
 }
 
 /**