D15025: Avoid crash with on scripted window teardown with threaded quick render loop

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REVISION SUMMARY
  Qt render loops behave quite differently to each other.
  
  KWin scripting as a workaround for another situation cleans
  (17553e5a1fb72a92dbbff3fa6bc7d55b8f1a0123 <https://phabricator.kde.org/R108:17553e5a1fb72a92dbbff3fa6bc7d55b8f1a0123>) 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 PLAN
  Ran QSG_RENDER_LOOP=threaded
  Read output with scenegraph logging rules on

REPOSITORY
  R108 KWin

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D15025

AFFECTED FILES
  scripting/scripting.cpp

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