iOS: Avoid RefCell and static mut (#4255)

* iOS: Refactor queued_gpu_redraws out from AppStateImpl

To allow AppStateImpl to be Copy, and to move redraws into the window in
the future.

* iOS AppState: Avoid RefCell and static mut

Instead, prefer Cell and Copy types, as those will never have crashes
on re-entrancy / if forgetting to make a state transition.

Author: madsmtm