Widgets: resize outdated backing stores on expose

An unexposed resize can leave a clean repaint manager with
buffers sized for the old window. The expose fast path flushed
that buffer without resizing, clipping Calculator on restore.
Require the backing store to match the current widget size.

Add an outdated-backing-store regression. Verify it fails
before the fix and passes afterward, then check three matched
Android rotation and keyguard restores with correct backing-
store size.

Pick-to: 6.12
Change-Id: I5d5ec7585e5894b844f13eb21c8aaa9e3333c456
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>