centerM cmds d2m d2mgs displayMmToModel displayMmToModel displayMmToModel displayMmToModelBox displayMmToModelBox distanceM dragging gestureState gestureType lastPt lastPtM MgMotion point point2 point2M pointM pressDrag startCenterM startDistanceM startPt startPt2 startPt2M startPtM switchGesture velocity view

startCenterM() Point2d MgMotion::startCenterM ( ) const 双指起始位置的中心点，模型坐标