TilesRendererBase: Add more "structure" for cached, per-engine fields

[gkjohnson]

• Ie add a "cached" (or similarly named) object to tiles in the "Base" class implementation and existing slots for canonical objects, like "scene" or "content"
• Sub classes like three.js can reliably set these fields so the base class can use them in function signatures - subclasses can otherwise set any other fields needed.