Bug 1636038 - Only rebuild the scene if the a displaylist or root pipeline was updated. r=gw

This reverts to the behavior before https://phabricator.services.mozilla.com/D71781 which introduced a talos regression.

Differential Revision: https://phabricator.services.mozilla.com/D74228

gfx/wr/webrender/src/scene_builder_thread.rs

@@ -607,7 +607,7 @@ impl SceneBuilderThread {
 
         let mut discard_frame_state_for_pipelines = Vec::new();
         let mut removed_pipelines = Vec::new();
-        let rebuild_scene = !txn.scene_ops.is_empty();
+        let mut rebuild_scene = false;
         for message in txn.scene_ops.drain(..) {
             match message {
                 SceneMsg::UpdateEpoch(pipeline_id, epoch) => {
@@ -633,6 +633,7 @@ impl SceneBuilderThread {
                     list_data,
                     preserve_frame_state,
                 } => {
+                    rebuild_scene = true;
                     let built_display_list =
                         BuiltDisplayList::from_data(list_data, list_descriptor);
                     let display_list_len = built_display_list.data().len();
@@ -668,6 +669,7 @@ impl SceneBuilderThread {
                     }
                 }
                 SceneMsg::SetRootPipeline(pipeline_id) => {
+                    rebuild_scene = true;
                     scene.set_root_pipeline_id(pipeline_id);
                 }
                 SceneMsg::RemovePipeline(pipeline_id) => {