Mouse Clicks
Button 1: Sets the cursor, drawing it in the new location, and marks the clicked vertex.
Button 2: Sets the cursor without drawing it in the new location and unmarks the clicked vertex.
Button 3: Clears marked vertices.

Mouse Controls for Zooming
Control+Button 1: Zooms in, doubling the zoom level
Control+Button 3: Zooms out, halving the zoom level