Four build 270 crash logs abort inside the GL driver
(GLDContextRec::flushContextInternal) from presentRenderbuffer, reached
from the immediate render queued by appDidBecomeActive/resumeRendering.
Only displayLinkFired checked isInBackground; renders dispatched
directly to the render queue could run, or present, after the app was
backgrounded again, where GPU work is not permitted.
Check isInBackground at the top of performRender and again before
presenting. PiP rendering is unaffected.