Client: Fix touch getting stuck after drag-and-drop

wl_touch.up is not sent by compositors when dragging, so release all touch
points when the drag ends.

Task-number: QTBUG-56187
Change-Id: I1c3d03c72e75a551355c50bb5d82433f5e2e35f0
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>