Make the ClipRenderContext optional.

This allows us to update the clip scroll tree but outside the context of
a render. It just skips the steps that are specific to rendering.

webrender/src/clip_node.rs

@@ -30,14 +30,16 @@ impl ClipNode {
         &mut self,
         device_pixel_scale: DevicePixelScale,
         clip_store: &mut ClipStore,
-        render_context: &mut ClipRenderContext,
+        render_context: &mut Option<ClipRenderContext>,
         clip_chains: &mut [ClipChain],
         spatial_nodes: &[SpatialNode],
     ) {
         let clip_sources = clip_store.get_mut(self.clip_sources_index);
-        clip_sources.update(render_context.gpu_cache,
-                            render_context.resource_cache,
-                            device_pixel_scale);
+        if let &mut Some(ref mut context) = render_context {
+            clip_sources.update(context.gpu_cache,
+                                context.resource_cache,
+                                device_pixel_scale);
+        }
         let spatial_node = &spatial_nodes[clip_sources.spatial_node_index.0];
 
         let (screen_inner_rect, screen_outer_rect) = clip_sources.get_screen_bounds(

webrender/src/clip_scroll_tree.rs

@@ -228,11 +228,15 @@ impl ClipScrollTree {
         screen_rect: &DeviceIntRect,
         device_pixel_scale: DevicePixelScale,
         clip_store: &mut ClipStore,
-        render_context: &mut ClipRenderContext,
+        render_context: &mut Option<ClipRenderContext>,
         pan: WorldPoint,
         scene_properties: &SceneProperties,
-    ) -> TransformPalette {
-        let mut transform_palette = TransformPalette::new(self.spatial_nodes.len());
+    ) -> Option<TransformPalette> {
+        let mut transform_palette = if render_context.is_some() {
+            Some(TransformPalette::new(self.spatial_nodes.len()))
+        } else {
+            None
+        };
         if self.spatial_nodes.is_empty() {
             return transform_palette;
         }
@@ -278,7 +282,7 @@ impl ClipScrollTree {
         node_index: SpatialNodeIndex,
         state: &mut TransformUpdateState,
         next_coordinate_system_id: &mut CoordinateSystemId,
-        transform_palette: &mut TransformPalette,
+        transform_palette: &mut Option<TransformPalette>,
         scene_properties: &SceneProperties,
     ) {
         // TODO(gw): This is an ugly borrow check workaround to clone these.
@@ -291,7 +295,9 @@ impl ClipScrollTree {
             };
 
             node.update(&mut state, next_coordinate_system_id, scene_properties);
-            node.push_gpu_data(transform_palette, node_index);
+            if let &mut Some(ref mut palette) = transform_palette {
+                node.push_gpu_data(palette, node_index);
+            }
 
             if node.children.is_empty() {
                 return;

webrender/src/frame_builder.rs

@@ -331,18 +331,18 @@ impl FrameBuilder {
         gpu_cache.begin_frame();
 
         let transform_palette = {
-            let mut clip_render_context = ClipRenderContext {
+            let clip_render_context = ClipRenderContext {
                 resource_cache,
                 gpu_cache,
             };
             clip_scroll_tree.update_tree(
                 &self.screen_rect.to_i32(),
                 device_pixel_scale,
                 &mut self.clip_store,
-                &mut clip_render_context,
+                &mut Some(clip_render_context),
                 pan,
                 scene_properties,
-            )
+            ).expect("Must get a palette since we provided a render context")
         };
 
         self.update_scroll_bars(clip_scroll_tree, gpu_cache);