auto merge of #2080 : mbrubeck/servo/2069-forget-tiles, r=larsbergstrom

This fixes a pixmap leak and task failure (#2069).

src/components/main/compositing/compositor.rs

@@ -394,6 +394,11 @@ impl IOCompositor {
 
             ContainerLayer::add_child_start(self.root_layer.clone(),
                                             ContainerLayerKind(new_layer.root_layer.clone()));
+
+            // Release all tiles from the layer before dropping it.
+            for layer in self.compositor_layer.mut_iter() {
+                layer.clear_all_tiles();
+            }
             self.compositor_layer = Some(new_layer);
         }
 

src/components/main/compositing/compositor_layer.rs

@@ -879,7 +879,7 @@ impl CompositorLayer {
                 }
 
                 // Send back all tiles to renderer.
-                child.get_mut_ref().child.clear();
+                child.get_mut_ref().child.clear_all_tiles();
 
                 self.build_layer_tree(graphics_context);
                 true
@@ -951,6 +951,15 @@ impl CompositorLayer {
         }
     }
 
+    /// Destroys tiles for this layer and all descendent layers, sending the buffers back to the
+    /// renderer to be destroyed or reused.
+    pub fn clear_all_tiles(&mut self) {
+        self.clear();
+        for kid in self.children.mut_iter() {
+            kid.child.clear_all_tiles();
+        }
+    }
+
     /// Destroys all tiles of all layers, including children, *without* sending them back to the
     /// renderer. You must call this only when the render task is destined to be going down;
     /// otherwise, you will leak tiles.