QMutableEventPoint::update(): update the touchpoint uniqueId too

Most touch events don't have uniqueIds, but it happens with TUIO
fiducial objects.

Fixes: QTBUG-115758
Change-Id: Ibacce255898ce63090bc5b888c12242838603dd5
Pick-to: 6.5 6.6
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/gui/kernel/qeventpoint.cpp

@@ -536,6 +536,7 @@ void QMutableEventPoint::update(const QEventPoint &other, QEventPoint &target)
     setEllipseDiameters(target, other.ellipseDiameters());
     setRotation(target, other.rotation());
     setVelocity(target, other.velocity());
+    setUniqueId(target, other.uniqueId()); // for TUIO
 }
 
 /*! \internal