Bug 1579235 - Part 1 - Support prefer compositor flag in wrench. r=nical

Add support to the yaml reader and writer to be able to specify
that a primitive should set the PREFER_COMPOSITOR_SURFACE flag.

This flag is not currently used, but in future will signal the
picture caching code to promote a primitive to draw on a native
compositor surface where possible.

Differential Revision: https://phabricator.services.mozilla.com/D62693

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1e1ffcbadfb2350ecda25b8752b3c07c2f2d0722

wrench/src/yaml_frame_reader.rs

@@ -1732,6 +1732,13 @@ impl YamlFrameReader {
                     flags.remove(PrimitiveFlags::IS_SCROLLBAR_CONTAINER);
                 }
             }
+            if let Some(prefer_compositor_surface) = item["prefer-compositor-surface"].as_bool() {
+                if prefer_compositor_surface {
+                    flags.insert(PrimitiveFlags::PREFER_COMPOSITOR_SURFACE);
+                } else {
+                    flags.remove(PrimitiveFlags::PREFER_COMPOSITOR_SURFACE);
+                }
+            }
 
             let mut info = CommonItemProperties {
                 clip_rect,

wrench/src/yaml_frame_writer.rs

@@ -205,6 +205,7 @@ fn common_node(v: &mut Table, clip_id_mapper: &mut ClipIdMapper, info: &CommonIt
     rect_node(v, "clip-rect", &info.clip_rect);
     bool_node(v, "backface-visible", info.flags.contains(PrimitiveFlags::IS_BACKFACE_VISIBLE));
     bool_node(v, "scrollbar-container", info.flags.contains(PrimitiveFlags::IS_SCROLLBAR_CONTAINER));
+    bool_node(v, "prefer-compositor-surface", info.flags.contains(PrimitiveFlags::PREFER_COMPOSITOR_SURFACE));
     bool_node(v, "scrollbar-thumb", info.flags.contains(PrimitiveFlags::IS_SCROLLBAR_THUMB));
 
     clip_and_scroll_node(v, clip_id_mapper, info.clip_id, info.spatial_id);