Make sure window thumbnails and Qt Quick resources are destroyed properly
Drops the doneCurrent as it was preventing proper cleanUp because no context was current when textures were deleted. Also avoid manipulating the context when Qt has the current one, as various Qt classes have guards around their cleanup handlers which rely on a current Qt context. Despite the comment the order of render control and view destruction needs to be switched as the QQuickWindow destructor calls into the render control to notify if of window destruction. BUG:478770 BUG:479846 FIXED-IN:6.0
(cherry picked from commit 203c4998)