On iOS, backgrounded applications are not permitted to execute GPU work.
See [1].  When backgrounding the application, we now tear down the
underlying iOS render surface, and re-create when the app returns to
foreground.

See [2] for a Technica Q&A on GL-related crashes when rendering is
attempted by a backgrounded application.

[1]: https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/ImplementingaMultitasking-awareOpenGLESApplication/ImplementingaMultitasking-awareOpenGLESApplication.html#//apple_ref/doc/uid/TP40008793-CH5-SW1
[2]: https://developer.apple.com/library/content/qa/qa1766/_index.html