Mouse event
A mouse event identifier is passed to the canvas OnEvent callback.
The state of the mouse buttons (and some keys) can be examined by
calling the following functions.
mouse-event-button
mouse-event-button-down
mouse-event-control-down
mouse-event-dragging
mouse-event-left-down
mouse-event-left-up
mouse-event-is-button
mouse-event-middle-down
mouse-event-middle-up
mouse-event-right-down
mouse-event-right-up
mouse-event-shift-down