Fixes two issues related to bevyengine#13208.

First, we ensure render resources for a window are always dropped first
to ensure that the `winit::Window` always drops on the main thread when
it is removed from `WinitWindows`. Previously, changes in bevyengine#12978 caused
the window to drop in the render world, causing issues.

We accomplish this by delaying despawning the window by a frame by
inserting a marker component `ClosingWindow` that indicates the window
has been requested to close and is in the process of closing. The
render world now responds to the equivalent `WindowClosing` event
rather than `WindowCloseed` which now fires after the render resources
are guarunteed to be cleaned up.

Secondly, fixing the above caused (revealed?) that additional events
were being delivered to the the event loop handler after exit had
already been requested: in my testing `RedrawRequested` and
`LoopExiting`. This caused errors to be reported try to send an
exit event on the close channel. There are two options here:
    - Guard the handler so no additional events are delivered once the
    app is exiting. I considered this but worried it might be
    confusing or bug prone if in the future someone wants to handle
   `LoopExiting` or some other event to clean-up while exiting.
    - Only send an exit signal if we are not already exiting. It
    doesn't appear to cause any problems to handle the extra events
    so this seems safer.
Fixing this also appears to have fixed bevyengine#13231.