Auto merge of #13848 - mrobinson:remove-layers, r=glennw

Remove concept of Layers from Servo

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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.

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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.