Fusion style: cleanup drawControl()

Cleanup drawControl():
 - don't create a copy of option->rect if not needed
 - don't calc unneeded variables on every call
 - don't calc colors twice

Task-number: QTBUG-129680
Change-Id: I4ad5c9c4dde42d9ca16ac38ad48b56b3844e0eb0
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>