Fix Flickable StopAtBounds drag over, back, over behavior.

A Flickable with StopAtBounds failed when:
1. position on a boundary.
Without lifting your finger:
2. attempt to drag beyond the boundary -> doesn't drag
3. drag back to initiate dragging
4. attempt to quickly drag beyond the boundary.

After 4, the view should be back on the boundary, but it could get
stuck a little short of the boundary.

Change-Id: I9bfbb4293f4d464bddb97c5c37e9bb91ed7d48e4
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>