This change makes system drag become initiated right after dragging
is detected when drag start distance from
QApplication::startDragDistance is 0. Otherwise the starting distance
is ignored and the value of 1 is implicitly used.
The startDragDistance of 0 is needed on WASM, as the native
dragstart event arrives exactly after the first mousemove event.
With this change, it is possible to set up drag correctly prior to
the native dragstart event arriving.
Fixes: QTBUG-114947
Change-Id: I112d97d251c9e9b1a39196ddcc39a37024b441f6
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>