Update CameraCache failure case + perf

[keveleigh]

Overview

Upgrades the "no main camera found, but at least one camera found in the scene" case to an error. There are too many assumptions MRTK would have to make to handle any other case, like auto-tagging existing camera.

Also adds a cache for IsOpaque in some hot loops, which can be expensive.

Changes

• Fixes: CameraCache.Main Can't Assign Existing Cameras if It Finds Camera.main to be null #9395 (essentially, at least, by upgrading the log to an error.)