Prioritize scene component initialization

Description: Currently, child entities initialize before parent entities. The scene is the most-parent/root "entity". Thus, all entities initialize before the scene. This means that scene components aren't initialized when entities are initialized. This might cause components having to register listeners to the scene to do things such as retrieve the canvas. We should initialize scene components first (for now, all of them, until we see a use case to not do so).

• A-Frame Version: master
• Platform/Device: all

Start by tracing the scene and child-entity initialization path to identify where scene components are initialized relative to entities. Verify the ordering across the relevant initialization flow, then add or update coverage so scene components are initialized before entity components and run the existing test suite.