BoundaryRule fails to enforce limits when property is set indirectly

This can be demonstrated with qtwebengine/tests/manual/quick/pdf/pdfPageView.qml: if you zoom in or out a long way (using e.g. control-mousewheel), the page may remain scrolled out of the viewport, despite this call to returnToBounds():

    PdfPageView {
        id: pageView
...
        onRenderScaleChanged: pageView.returnToBounds()
...
        BoundaryRule on y {
            id: bry
            objectName: "bry"
            minimumOvershoot: 100
            maximumOvershoot: 100
            minimum: Math.min(0, root.height - pageView.height)
            maximum: 0
        }

        function returnToBounds() {
            bry.returnToBounds()
            brx.returnToBounds()
        }
    }

The reason is that QQuickBoundaryRule::returnToBounds() checks currentOvershoot() which simply returns d->currentOvershoot; but zooming only changed the Image's width and height, and therefore indirectly changed the size of the PdfPageView: the QML code did not directly set y as a result of zooming, so BoundaryRule did not intercept it, and therefore doesn't know that there is any overshoot. Both currentOvershoot and targetValue are outdated at that point.

Perhaps when the user calls returnToBounds(), we should assume there is a good reason, and be more thorough about checking. The way to fix it seems to be

    d->targetValue = d->easedOvershoot(d->property.read().toReal());