Fix ambiguous type conversion options (QPointF vs QDoubleVector)

qdeclarativegeomapgesturearea.cpp: In member function 'void QDeclarativeGeoMapGestureArea::startTwoTouchPoints()':
qdeclarativegeomapgesturearea.cpp:834:66: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: [enabled by default]
     QPointF startPos = (sceneStartPoint1_ + sceneStartPoint2_) * 0.5;
                                                                  ^
In file included from /home/ablasche/dev/qt/androidqt/qtbase/include/QtCore/qpoint.h:1:0,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtCore/../../src/corelib/tools/qrect.h:46,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtCore/qrect.h:1,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtGui/../../src/gui/painting/qregion.h:46,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtGui/qregion.h:1,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtGui/../../src/gui/kernel/qevent.h:47,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtGui/qevent.h:1,
    from /home/ablasche/dev/qt/androidqt/qtbase/include/QtGui/QTouchEvent:1,
    from qdeclarativegeomapgesturearea_p.h:46,
    from qdeclarativegeomapgesturearea.cpp:42:
/home/ablasche/dev/qt/androidqt/qtbase/include/QtCore/../../src/corelib/tools/qpoint.h:357:39: note: candidate 1: constexpr const QPointF operator*(const QPointF&, qreal)
 Q_DECL_CONSTEXPR inline const QPointF operator*(const QPointF &p, qreal c)
                                      ^
In file included from /home/ablasche/dev/qt/androidqt/qtbase/include/QtPositioning/5.3.0/QtPositioning/private/qdoublevector2d_p.h:1:0,
    from ../../location/maps/qgeomap_p.h:59,
    from qdeclarativegeomap_p.h:73,
    from qdeclarativegeomapgesturearea.cpp:43:
/home/ablasche/dev/qt/androidqt/qtbase/include/QtPositioning/5.3.0/QtPositioning/private/../../../../../../qtlocation/src/positioning/qdoublevector2d_p.h:208:30: note: candidate 2: const QDoubleVector2D operator*(const QDoubleVector2D&, double)
 inline const QDoubleVector2D operator*(const QDoubleVector2D &vector, double factor)
                              ^

Change-Id: Ic9f56a14f5222b54f24443f3546b97f88a3dcf53
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>