Fix OffscreenBuffer leak

Fixes: 27941148

Make OffscreenBuffer lifecycle an explicit (and tested) contract between
FrameBuilder and BakedOpRenderer, entirely separate from dispatch. This
makes it safe to reject any rendering work via overdraw content
rejection (before it gets to a BakedOpDispatcher).

Adds a couple tests around OffscreenBuffer leaks, and switches
OffscreenBuffer tests to RENDERTHREAD_TEST macro, as appropriate.

Change-Id: Id114b835d042708ae921028fb4b17e5fa485fe64