Bug 1612440 - Move some scene-related transaction handling code to the scene builder thread. r=gw

The goal is to move this code out of the render backend to avoid requiring the messages to transit through this thread. Other similar changes are in progress.

Differential Revision: https://phabricator.services.mozilla.com/D71781

UltraBlame original commit: 7675477d0507096eaaaeb916ed8b5ec44c5b6db1

Author: marco-c