Flatten the display list in the API

This changes brings the WebRender API more into line with the Servo
display list and also the frames generated inside WebRender itself.
Most intermediate display list representations are removed as well as
their place on the resource cache. Perhaps most notably, stacking
contexts are started via a PushStackingContext display item and ended
via a PopStackingContext item. Additionally, the flatten pass has been
significantly simplified to account for these changes.

sample/src/main.rs

@@ -1,4 +1,3 @@
-extern crate app_units;
 extern crate webrender;
 extern crate glutin;
 extern crate gleam;
@@ -9,9 +8,8 @@ use euclid::{Size2D, Point2D, Rect, Matrix4D};
 use gleam::gl;
 use std::path::PathBuf;
 use std::ffi::CStr;
-use webrender_traits::{PipelineId, StackingContextId, DisplayListId};
-use webrender_traits::{AuxiliaryListsBuilder, Epoch, ColorF, GlyphInstance};
-use webrender_traits::{ImageFormat, RendererKind};
+use webrender_traits::{AuxiliaryListsBuilder, ColorF, Epoch, GlyphInstance};
+use webrender_traits::{ImageFormat, PipelineId, RendererKind};
 use std::fs::File;
 use std::io::Read;
 use std::env;
@@ -36,54 +34,10 @@ impl Notifier {
 }
 
 pub struct WebRenderFrameBuilder {
-    pub stacking_contexts: Vec<(StackingContextId, webrender_traits::StackingContext)>,
-    pub display_lists: Vec<(DisplayListId, webrender_traits::BuiltDisplayList)>,
-    pub auxiliary_lists_builder: AuxiliaryListsBuilder,
     pub root_pipeline_id: PipelineId,
     pub next_scroll_layer_id: usize,
 }
 
-impl WebRenderFrameBuilder {
-    pub fn new(root_pipeline_id: PipelineId) -> WebRenderFrameBuilder {
-        WebRenderFrameBuilder {
-            stacking_contexts: vec![],
-            display_lists: vec![],
-            auxiliary_lists_builder: AuxiliaryListsBuilder::new(),
-            root_pipeline_id: root_pipeline_id,
-            next_scroll_layer_id: 0,
-        }
-    }
-
-    pub fn add_stacking_context(&mut self,
-                                api: &mut webrender_traits::RenderApi,
-                                pipeline_id: PipelineId,
-                                stacking_context: webrender_traits::StackingContext)
-                                -> StackingContextId {
-        assert!(pipeline_id == self.root_pipeline_id);
-        let id = api.next_stacking_context_id();
-        self.stacking_contexts.push((id, stacking_context));
-        id
-    }
-
-    pub fn add_display_list(&mut self,
-                            api: &mut webrender_traits::RenderApi,
-                            display_list: webrender_traits::BuiltDisplayList,
-                            stacking_context: &mut webrender_traits::StackingContext)
-                            -> DisplayListId {
-        let id = api.next_display_list_id();
-        stacking_context.display_lists.push(id);
-        self.display_lists.push((id, display_list));
-        id
-    }
-
-    pub fn next_scroll_layer_id(&mut self) -> webrender_traits::ScrollLayerId {
-        let scroll_layer_id = webrender_traits::ServoScrollRootId(self.next_scroll_layer_id);
-        self.next_scroll_layer_id += 1;
-        webrender_traits::ScrollLayerId::new(self.root_pipeline_id, 0, scroll_layer_id)
-    }
-
-}
-
 impl webrender_traits::RenderNotifier for Notifier {
     fn new_frame_ready(&mut self) {
         self.window_proxy.wakeup_event_loop();
@@ -144,7 +98,7 @@ fn main() {
     };
 
     let (mut renderer, sender) = webrender::renderer::Renderer::new(opts);
-    let mut api = sender.create_api();
+    let api = sender.create_api();
 
 //     let font_path = "/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf";
 //     let font_bytes = load_file(font_path);
@@ -157,11 +111,15 @@ fn main() {
     let epoch = Epoch(0);
     let root_background_color = ColorF::new(0.3, 0.0, 0.0, 1.0);
 
-    let mut frame_builder = WebRenderFrameBuilder::new(pipeline_id);
-    let root_scroll_layer_id = frame_builder.next_scroll_layer_id();
+    let mut auxiliary_lists_builder = AuxiliaryListsBuilder::new();
+    let mut builder = webrender_traits::DisplayListBuilder::new();
+
+    let root_scroll_layer_id =
+        webrender_traits::ScrollLayerId::new(pipeline_id, 0,
+                                             webrender_traits::ServoScrollRootId(0));
 
     let bounds = Rect::new(Point2D::new(0.0, 0.0), Size2D::new(width as f32, height as f32));
-    let mut sc =
+    builder.push_stacking_context(
         webrender_traits::StackingContext::new(Some(root_scroll_layer_id),
                                                webrender_traits::ScrollPolicy::Scrollable,
                                                bounds,
@@ -172,9 +130,7 @@ fn main() {
                                                true,
                                                webrender_traits::MixBlendMode::Normal,
                                                Vec::new(),
-                                               &mut frame_builder.auxiliary_lists_builder);
-
-    let mut builder = webrender_traits::DisplayListBuilder::new();
+                                               &mut auxiliary_lists_builder));
 
     let clip_region = {
         let rect = Rect::new(Point2D::new(100.0, 100.0), Size2D::new(100.0, 100.0));
@@ -189,7 +145,7 @@ fn main() {
         webrender_traits::ClipRegion::new(&bounds,
                                           vec![complex],
                                           Some(mask),
-                                          &mut frame_builder.auxiliary_lists_builder)
+                                          &mut auxiliary_lists_builder)
     };
 
     builder.push_rect(Rect::new(Point2D::new(100.0, 100.0), Size2D::new(100.0, 100.0)),
@@ -270,19 +226,15 @@ fn main() {
 //                       Au::from_px(0),
 //                       &mut frame_builder.auxiliary_lists_builder);
 
-    frame_builder.add_display_list(&mut api, builder.finalize(), &mut sc);
-    let sc_id = frame_builder.add_stacking_context(&mut api, pipeline_id, sc);
-
-    api.set_root_stacking_context(sc_id,
-                                  root_background_color,
-                                  epoch,
-                                  pipeline_id,
-                                  Size2D::new(width as f32, height as f32),
-                                  frame_builder.stacking_contexts,
-                                  frame_builder.display_lists,
-                                  frame_builder.auxiliary_lists_builder
-                                               .finalize());
+    builder.pop_stacking_context();
 
+    api.set_root_display_list(
+        root_background_color,
+        epoch,
+        pipeline_id,
+        Size2D::new(width as f32, height as f32),
+        builder.finalize(),
+        auxiliary_lists_builder.finalize());
     api.set_root_pipeline(pipeline_id);
 
     for event in window.wait_events() {