Bug 1607836 - Preallocate picture task dependency vectors. r=gw

Unlike other types of render tasks, pictures can have hundreds of dependencies. The dependency vector is re-built every frame, leading to a lot of vector re-allocations in some pages.

Depends on D60151

Differential Revision: https://phabricator.services.mozilla.com/D60182

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/dfe4fa983d2b1eaaa3db2141a5e1c0f5ae58d5d7

webrender/src/frame_builder.rs

@@ -438,6 +438,7 @@ impl FrameBuilder {
 
         let pic = &mut scene.prim_store.pictures[scene.root_pic_index.0];
         pic.restore_context(
+            ROOT_SURFACE_INDEX,
             prim_list,
             pic_context,
             pic_state,

webrender/src/picture.rs

@@ -3277,6 +3277,10 @@ pub struct PicturePrimitive {
 
     /// The config options for this picture.
     pub options: PictureOptions,
+
+    /// Keep track of the number of render tasks dependencies to pre-allocate
+    /// the dependency array next frame.
+    num_render_tasks: usize,
 }
 
 impl PicturePrimitive {
@@ -3419,6 +3423,7 @@ impl PicturePrimitive {
             tile_cache,
             options,
             segments_are_valid: false,
+            num_render_tasks: 0,
         }
     }
 
@@ -3461,6 +3466,9 @@ impl PicturePrimitive {
             return None;
         }
 
+        let task_id = frame_state.surfaces[parent_surface_index.0].render_tasks.unwrap().port;
+        frame_state.render_tasks[task_id].children.reserve(self.num_render_tasks);
+
         // Extract the raster and surface spatial nodes from the raster
         // config, if this picture establishes a surface. Otherwise just
         // pass in the spatial node indices from the parent context.
@@ -4225,6 +4233,7 @@ impl PicturePrimitive {
 
     pub fn restore_context(
         &mut self,
+        parent_surface_index: SurfaceIndex,
         prim_list: PrimitiveList,
         context: PictureContext,
         state: PictureState,
@@ -4235,6 +4244,9 @@ impl PicturePrimitive {
             frame_state.pop_dirty_region();
         }
 
+        let task_id = frame_state.surfaces[parent_surface_index.0].render_tasks.unwrap().port;
+        self.num_render_tasks = frame_state.render_tasks[task_id].children.len();
+
         self.prim_list = prim_list;
         self.state = Some(state);
     }

webrender/src/prim_store/mod.rs

@@ -2732,6 +2732,7 @@ impl PrimitiveStore {
                 // Restore the dependencies (borrow check dance)
                 self.pictures[pic_context_for_children.pic_index.0]
                     .restore_context(
+                        pic_context.surface_index,
                         prim_list,
                         pic_context_for_children,
                         pic_state_for_children,