Fix TapHandler so that it actually registers a tap

This bug caused all quick examples that used the
shared\LauncherList.qml to be broken.

In QtGui, QSinglePointEvent will construct itself with a point id of 0
if there is a valid point, and with a point id of -1 if the point is
invalid (the default constructor does the latter).
However, QQuickSinglePointHandler::wantsPointerEvent() did not agree
with that, because it assumed that a point id of 0 meant
uninitialized/invalid point.
The fix is to change QQuickSinglePointHandler::wantsPointerEvent() and
QQuickHandlerPoint so that it assumes that the id -1 is now an invalid
point, (instead of 0)

Change-Id: I8c9683dfe06ebb77c5342a26f08174b67e7cbd90
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/quick/handlers/qquickhandlerpoint.cpp

@@ -82,7 +82,7 @@ void QQuickHandlerPoint::localize(QQuickItem *item)
 
 void QQuickHandlerPoint::reset()
 {
-    m_id = 0;
+    m_id = -1;
     m_device = QPointingDevice::primaryPointingDevice();
     m_uniqueId = QPointingDeviceUniqueId();
     m_position = QPointF();
@@ -167,7 +167,7 @@ void QQuickHandlerPoint::reset(const QVector<QQuickHandlerPoint> &points)
         pressureSum += point.pressure();
         ellipseDiameterSum += point.ellipseDiameters();
     }
-    m_id = 0;
+    m_id = -1;
     m_device = nullptr;
     m_uniqueId = QPointingDeviceUniqueId();
     // all points are required to be from the same event, so pressed buttons and modifiers should be the same

src/quick/handlers/qquickhandlerpoint_p.h

@@ -100,7 +100,7 @@ public:
     void reset(const QVector<QQuickHandlerPoint> &points);
 
 private:
-    int m_id = 0;
+    int m_id = -1;
     const QPointingDevice *m_device = QPointingDevice::primaryPointingDevice();
     QPointingDeviceUniqueId m_uniqueId;
     Qt::MouseButtons m_pressedButtons = Qt::NoButton;

src/quick/handlers/qquicksinglepointhandler.cpp

@@ -75,7 +75,7 @@ bool QQuickSinglePointHandler::wantsPointerEvent(QQuickPointerEvent *event)
     if (!QQuickPointerDeviceHandler::wantsPointerEvent(event))
         return false;
 
-    if (d->pointInfo.id()) {
+    if (d->pointInfo.id() != -1) {
         // We already know which one we want, so check whether it's there.
         // It's expected to be an update or a release.
         // If we no longer want it, cancel the grab.
@@ -125,7 +125,7 @@ bool QQuickSinglePointHandler::wantsPointerEvent(QQuickPointerEvent *event)
             chosen->setAccepted();
         }
     }
-    return d->pointInfo.id();
+    return d->pointInfo.id() != -1;
 }
 
 void QQuickSinglePointHandler::handlePointerEventImpl(QQuickPointerEvent *event)