Touch to mouse events can have wrong coordinates when filtered from children

if I have an item (easily reproduceable with a C++ element) that filters child mouse events that contains a Flickable, when the Flickable content position is not at 0,0 the filtered events arrive with a different position depending it's a real click or a synthesized one from touch events, in that case the click event arrives with a local position relative to the Flickable instead of being relative to the child item.

attached is the smallest test case i could reproduce. It has a C++ element instead of a MouseArea, but the values arriving to it do indeed suggest there is a problem.
A proposed patch is at https://codereview.qt-project.org/#/c/114424/