Remove concept of Layers from Servo

Layers were a feature of the legacy drawing path. If we re-add them at
some point, it probably makes more sense to make them a product of
display list inspection.

This change also remove a bunch of dead painting code.

components/compositing/compositor.rs

@@ -12,7 +12,7 @@ use euclid::point::TypedPoint2D;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 use gfx_traits::{DevicePixel, LayerPixel, StackingContextId};
-use gfx_traits::{Epoch, FrameTreeId, FragmentType, LayerId};
+use gfx_traits::{Epoch, FrameTreeId, FragmentType};
 use gleam::gl;
 use gleam::gl::types::{GLint, GLsizei};
 use image::{DynamicImage, ImageFormat, RgbImage};
@@ -503,9 +503,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.title_for_main_frame();
             }
 
-            (Msg::ScrollFragmentPoint(pipeline_id, layer_id, point, _),
+            (Msg::ScrollFragmentPoint(pipeline_id, point, _),
              ShutdownState::NotShuttingDown) => {
-                self.scroll_fragment_to_point(pipeline_id, layer_id, point);
+                self.scroll_fragment_to_point(pipeline_id, point);
             }
 
             (Msg::MoveTo(point),
@@ -770,7 +770,6 @@ impl<Window: WindowMethods> IOCompositor<Window> {
 
     fn scroll_fragment_to_point(&mut self,
                                 _pipeline_id: PipelineId,
-                                _layer_id: LayerId,
                                 _point: Point2D<f32>) {
         println!("TODO: Support scroll_fragment_to_point again");
     }

components/compositing/compositor_thread.rs

@@ -8,7 +8,6 @@ use SendableFrameTree;
 use compositor::CompositingReason;
 use euclid::point::Point2D;
 use euclid::size::Size2D;
-use gfx_traits::LayerId;
 use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::{Image, Key, KeyModifiers, KeyState, PipelineId};
 use profile_traits::mem;
@@ -72,7 +71,7 @@ pub enum Msg {
     ShutdownComplete,
 
     /// Scroll a page in a window
-    ScrollFragmentPoint(PipelineId, LayerId, Point2D<f32>, bool),
+    ScrollFragmentPoint(PipelineId, Point2D<f32>, bool),
     /// Alerts the compositor that the current page has changed its title.
     ChangePageTitle(PipelineId, Option<String>),
     /// Alerts the compositor that the current page has changed its URL.

components/constellation/constellation.rs

@@ -933,11 +933,10 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 self.handle_alert(pipeline_id, message, sender);
             }
 
-            FromScriptMsg::ScrollFragmentPoint(pipeline_id, layer_id, point, smooth) => {
+            FromScriptMsg::ScrollFragmentPoint(pipeline_id, point, smooth) => {
                 self.compositor_proxy.send(ToCompositorMsg::ScrollFragmentPoint(pipeline_id,
-                                                               layer_id,
-                                                               point,
-                                                               smooth));
+                                                                                point,
+                                                                                smooth));
             }
 
             FromScriptMsg::GetClientWindow(send) => {