gpus_ReturnNotPermittedKillClient error occuring in iOS background-aware applications

[msmollin]

We received some feedback in the Mapzen iOS SDK causing crashes for applications going into the background. This seems to stem from GLKitViewController not pausing its rendering (like I thought it did) when going into and out of background. On reflection this does make some sense as it would be silly for Apple to assume it knows the state of the render pipeline.

To resolve this, we should follow the guidelines outlined in https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/ImplementingaMultitasking-awareOpenGLESApplication/ImplementingaMultitasking-awareOpenGLESApplication.html#//apple_ref/doc/uid/TP40008793-CH5-SW1

tangrams/tangram-es#1708 has been opened in the upstream Tangram-es repo as that is probably the correct place to implement this. However, we can probably implement something in the iOS-SDK sooner if necessary.

[msmollin]

After discussion we'll implement a fix for this in the iOS SDK and then when Tangram-es 0.9 lands, we will revert this fix for the more permanent fix.