Fix scrolldown arrow not showing on popup for QMenu

Fix scrolldown arrow not showing on popup for QMenu

Scrolldown arrow was not shown when a taller than screen QMenu was
opened because the check to draw it used the size that was already
adjusted to the screen.

Fixed by using the actual menu size in the check. Also fixed the case
where the menu was scrolled, closed, and reopened, in which case the
size hint would return incorrect cached value. This led to scrolldown
arrow not being shown in case the menu was previously fully scrolled
down.

Task-number: QTBUG-27445
Change-Id: Icd8d774071662a9317b3ac53cb05b31cadba96ff
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>