Move more details into DisplayListBuilder

Bookkeeping for pipeline_id and the current scroll layer id is logic
that is shared among all clients. We can just move this into
DisplayListBuilder.

sample/src/main.rs

@@ -32,11 +32,6 @@ impl Notifier {
     }
 }
 
-pub struct WebRenderFrameBuilder {
-    pub root_pipeline_id: PipelineId,
-    pub next_scroll_layer_id: usize,
-}
-
 impl webrender_traits::RenderNotifier for Notifier {
     fn new_frame_ready(&mut self) {
         self.window_proxy.wakeup_event_loop();
@@ -101,11 +96,11 @@ fn main() {
     let notifier = Box::new(Notifier::new(window.create_window_proxy()));
     renderer.set_render_notifier(notifier);
 
-    let pipeline_id = PipelineId(0, 0);
     let epoch = Epoch(0);
     let root_background_color = ColorF::new(0.3, 0.0, 0.0, 1.0);
 
-    let mut builder = webrender_traits::DisplayListBuilder::new();
+    let pipeline_id = PipelineId(0, 0);
+    let mut builder = webrender_traits::DisplayListBuilder::new(pipeline_id);
 
     let bounds = Rect::new(Point2D::new(0.0, 0.0), Size2D::new(width as f32, height as f32));
     builder.push_stacking_context(webrender_traits::ScrollPolicy::Scrollable,
@@ -205,7 +200,6 @@ fn main() {
     api.set_root_display_list(
         root_background_color,
         epoch,
-        pipeline_id,
         Size2D::new(width as f32, height as f32),
         builder);
     api.set_root_pipeline(pipeline_id);

webrender_traits/src/api.rs

@@ -165,9 +165,9 @@ impl RenderApi {
     pub fn set_root_display_list(&self,
                                  background_color: ColorF,
                                  epoch: Epoch,
-                                 pipeline_id: PipelineId,
                                  viewport_size: Size2D<f32>,
                                  builder: DisplayListBuilder) {
+        let pipeline_id = builder.pipeline_id;
         let (display_list, auxiliary_lists) = builder.finalize();
         let msg = ApiMsg::SetRootDisplayList(background_color,
                                              epoch,

webrender_traits/src/display_list.rs

@@ -13,8 +13,8 @@ use {DisplayItem, DisplayListMode, FilterOp, YuvColorSpace};
 use {FontKey, GlyphInstance, GradientDisplayItem, GradientStop, IframeDisplayItem};
 use {ImageDisplayItem, ImageKey, ImageMask, ImageRendering, ItemRange, MixBlendMode, PipelineId};
 use {PushScrollLayerItem, PushStackingContextDisplayItem, RectangleDisplayItem, ScrollLayerId};
-use {ScrollPolicy, SpecificDisplayItem, StackingContext, TextDisplayItem, WebGLContextId};
-use {WebGLDisplayItem, YuvImageDisplayItem};
+use {ScrollPolicy, ServoScrollRootId, SpecificDisplayItem, StackingContext, TextDisplayItem};
+use {WebGLContextId, WebGLDisplayItem, YuvImageDisplayItem};
 
 impl BuiltDisplayListDescriptor {
     pub fn size(&self) -> usize {
@@ -49,14 +49,18 @@ pub struct DisplayListBuilder {
     pub mode: DisplayListMode,
     pub list: Vec<DisplayItem>,
     auxiliary_lists_builder: AuxiliaryListsBuilder,
+    pub pipeline_id: PipelineId,
+    next_scroll_layer_id: usize,
 }
 
 impl DisplayListBuilder {
-    pub fn new() -> DisplayListBuilder {
+    pub fn new(pipeline_id: PipelineId) -> DisplayListBuilder {
         DisplayListBuilder {
             mode: DisplayListMode::Default,
             list: Vec::new(),
             auxiliary_lists_builder: AuxiliaryListsBuilder::new(),
+            pipeline_id: pipeline_id,
+            next_scroll_layer_id: 0,
         }
     }
 
@@ -292,10 +296,13 @@ impl DisplayListBuilder {
     pub fn push_scroll_layer(&mut self,
                              clip: Rect<f32>,
                              content_size: Size2D<f32>,
-                             id: ScrollLayerId) {
+                             scroll_root_id: ServoScrollRootId) {
+        let scroll_layer_id = self.next_scroll_layer_id;
+        self.next_scroll_layer_id += 1;
+
         let item = PushScrollLayerItem {
             content_size: content_size,
-            id: id,
+            id: ScrollLayerId::new(self.pipeline_id, scroll_layer_id, scroll_root_id),
         };
 
         let item = DisplayItem {