[Monocle] Keep render future as a member var for #1781.
[Monocle] Wait for render future on page item dtor for #1781.
[Monocle] Explicitly clear the scene on remove, closes #1781.
[Monocle] Moved threaded render initiation to RequestThreadedRender().

Also, capture scale values on render time.
[Monocle] Return scale information.
[Monocle] Better stale render handling.

1. Avoid re-rendering if the same page is already rendering.
2. Check and re-render if the scale has changed since render request.