Like other QWindow properties we can just store it, and the platform
window should pick it up on creation like other properties.
[ChangeLog][QtGui][QWindow] setMask() no longer requires the window
to be created to have an effect; it can be set at any time.
Change-Id: I55b616363801b770bd61bda5325b443013b99866
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>