WheelHandler may need limits

BoundaryRule is useful in many cases; but when using property: "scale", you might rescale a scalable image by setting sourceSize when the handler's active property changes to false; so it's difficult to prevent the user from continuing to roll the wheel to scale above or below some arbitrary limit (to prevent the item from disappearing by being scaled too small, or running into the texture limit, etc.) The map.qml example ought to demonstrate how to limit scaling. And if PdfPageView is going to keep doing its own scaling, it needs to apply this kind of limit too.