Provide a way to suspend the renderer when running in background

## What problem does this solve or what need does it fill?

Rendering can consume power unnecessarily when running in the background in a multitasking OS. For this reason, mobile OSes such as iOS do not allow use of the GPU by background tasks.

## What solution would you like?

I would like a way to suspend the renderer, or maybe plugins in general, in some states.

## What alternative(s) have you considered?

I have considered serializing state and exiting the main loop, but this isn't supported for iOS.

## Additional context

Without this feature, if the app uses the GPU on iOS, it will be killed with this message:

> Execution of the command buffer was aborted due to an error during execution. Insufficient Permission (to submit GPU work from background) (IOAF code 6)