Avoid crash with on scripted window teardown with threaded quick render loop

Summary:
Qt render loops behave quite differently to each other.

KWin scripting as a workaround for another situation cleans
(17553e5a) handles tracking of script
windows by deleting the underlying window handle on hide.

This currently happens before the window gets the hideEvent.

Arguably this is a quirk with Qt, but in the current state:

- QSGThreadedRenderLoop deletes the platform window and cleans up
- We then get the hide() event. This no-ops because there's no window.
   (else branch of     case WM_TryRelease in qsgthreadedrenderloop.cpp)
- We carry on rendering animations despite having no platform
- undefined behaviour

Normally this isn't a problem as typically destruction of the platform window
happens only when a window is being deleted, we're messing with Qt
internals here.

If we make sure the QHideEvent is processed by the render loop first,
things seem fine.

BUG: 397767

Test Pl...