avm1: Return proper root object for Avm1::root_object

Was returning start_clip instead. Also call
DisplayObject::post_instantiation for root after it's created in
Player::new.

core/src/avm1.rs

@@ -986,7 +986,7 @@ impl<'gc> Avm1<'gc> {
 
     /// Obtain the value of `_root`.
     pub fn root_object(&self, context: &mut ActionContext<'_, 'gc, '_>) -> Value<'gc> {
-        context.start_clip.read().object()
+        context.root.read().object()
     }
 
     /// Obtain the value of `_global`.

core/src/player.rs

@@ -127,6 +127,13 @@ impl<Audio: AudioBackend, Renderer: RenderBackend, Navigator: NavigatorBackend>
             is_mouse_down: false,
         };
 
+        player.gc_arena.mutate(|gc_context, gc_root| {
+            gc_root
+                .root
+                .write(gc_context)
+                .post_instantiation(gc_context, gc_root.root)
+        });
+
         player.build_matrices();
         player.preload();