Like the QPolygon code. This fixes a mistake in failing to clear the
list before de-marshalling in operator>>.
Updated most of the QDataStream tests to have data in the objects
they're streaming into, to ensure that the stream overwrites everything.
Fixes: QTBUG-122684
Task-number: QTBUG-122704
Pick-to: 5.15 6.5 6.6 6.7
Change-Id: I01ec3c774d9943adb903fffd17b6920c72f5042b
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
1ebee8980b