qt: fix crash on close on wayland with Qt < 6.9.0

QWindow's wl_surface gets deleted when QWindow::hide() is called.

This is an exceptional behavior, and can be considered a Qt bug because destroying the window resources is tied to QWindow::destroy(), calling QWindow::hide() should not destroy the window's resources.

Regression since !6902 (merged).

As a side note, !6902 (merged) seems to behave really well on Windows nightlies.

Request review @chub.