Move hit testing information out of WebRender

Store hit testing information in a data structure that sits alongside
the display list in the compositor. This will allow the compositor to
store more information per-node. The data structure also takes care of
de-duplicating information between successive display list entries. In
the future, the data structure can be even more aggressive in producing
smaller side hit testing lists, if necessary.

components/script_traits/compositor.rs

@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Defines data structures which are consumed by the Compositor.
+
+use embedder_traits::Cursor;
+
+/// Information that Servo keeps alongside WebRender display items
+/// in order to add more context to hit test results.
+#[derive(Debug, Deserialize, Serialize)]
+pub struct HitTestInfo {
+    /// The id of the node of this hit test item.
+    pub node: u64,
+
+    /// The cursor of this node's hit test item.
+    pub cursor: Option<Cursor>,
+}
+
+/// A data structure which stores compositor-side information about
+/// display lists sent to the compositor.
+/// by a WebRender display list.
+#[derive(Debug, Default, Deserialize, Serialize)]
+pub struct CompositorDisplayListInfo {
+    /// An array of `HitTestInfo` which is used to store information
+    /// to assist the compositor to take various actions (set the cursor,
+    /// scroll without layout) using a WebRender hit test result.
+    pub hit_test_info: Vec<HitTestInfo>,
+}
+
+impl CompositorDisplayListInfo {
+    /// Add or re-use a duplicate HitTestInfo entry in this `CompositorHitTestInfo`
+    /// and return the index.
+    pub fn add_hit_test_info(&mut self, node: u64, cursor: Option<Cursor>) -> usize {
+        if let Some(last) = self.hit_test_info.last() {
+            if node == last.node && cursor == last.cursor {
+                return self.hit_test_info.len() - 1;
+            }
+        }
+
+        self.hit_test_info.push(HitTestInfo { node, cursor });
+        self.hit_test_info.len() - 1
+    }
+}