Sets the zoom (held within MinZoom and MaxZoom), keeping the page point under the anchor where it is unless the scroll range stops it (ZoomAt(in Viewport, Double, Double, Double)). The step shows at once from the tiles on the surface; outside a gesture it starts (or extends) a zoom that settles after SettleDelay.
public Viewport ZoomTo(double zoom, double anchorX, double anchorY)
The zoom in pixels per point.
The anchor's viewport x.
The anchor's viewport y.
Spec #33 viewer capability 2.