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/script/script_thread.rs

@@ -57,7 +57,6 @@ use dom::window::{ReflowReason, Window};
 use dom::worker::TrustedWorkerAddress;
 use euclid::Rect;
 use euclid::point::Point2D;
-use gfx_traits::LayerId;
 use hyper::header::{ContentType, Headers, HttpDate, LastModified};
 use hyper::header::ReferrerPolicy as ReferrerPolicyHeader;
 use hyper::method::Method;
@@ -1883,10 +1882,7 @@ impl ScriptThread {
         let point = Point2D::new(rect.origin.x.to_nearest_px() as f32,
                                  rect.origin.y.to_nearest_px() as f32);
 
-        let message = ConstellationMsg::ScrollFragmentPoint(pipeline_id,
-                                                            LayerId::null(),
-                                                            point,
-                                                            false);
+        let message = ConstellationMsg::ScrollFragmentPoint(pipeline_id, point, false);
         self.constellation_chan.send(message).unwrap();
     }