Avoid accidental creation of backing stores for offscreen surfaces

Aurorae decorations and several effects are powered by QQuickWindows
that render into offscreen surfaces.

Iterating through all windows and then calling winId() will create a
platform window including for our Aurorae decorations.

Not only is this wasteful but it caused an issue with resizing windows.
QWindow code updates the internal state directly if there's no
underlying platform window, if there is it delegates responsiblity to
the backend. Our own QPA didn't update geometry whilst hidden.

The result of that is Aurorae decorations "randomly" stop resizing
correctly as the contentItem stops resizing to the window size.

BUG: 465790
(cherry picked from commit 82b2fa37)