We’ve already set up emscripten_set_main_loop_arg to
call the main loop callback using requestAnimationFrame.

Pause main loop updates when no updates have been requested.
Restart when requestUpdate() is called again.

Applications can now be completely idle when there
are no updates, or animate continuously if requestUpdate()
is called at the end of the paint event.

Change-Id: I1764b09a698f2d98a0f0e008d93e553f04831d4f
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>