Custom tool to use the mouse to zoom in or out on an image window.

This tool allows the user to zoom in or out on a point on the image window interactively by clicking the mouse on the point of interest. When invoked on a McGraphOverlay that is being displayed in an IMcView, the tool shows a magnifying glass icon whenever the cursor passes into the viewed client window. When the left mouse button is pressed, then the IMcView.Magnification property is doubled. When shift-left button is pressed, the IMcView.Magnification property is halved.