Return the starting position of EVENT.
EVENT should be a click, drag, or key press event.
If it is a key press event, the return value has the form
(WINDOW POS (0 . 0) 0)
If it is a click or drag event, it has the form
(WINDOW AREA-OR-POS (X . Y) TIMESTAMP OBJECT POS (COL . ROW)
IMAGE (DX . DY) (WIDTH . HEIGHT))
The `posn-' functions access elements of such lists.
For more information, see Info node `(elisp)Click Events'.
If EVENT is a mouse or key press or a mouse click, this is the
position of the event. If EVENT is a drag, this is the starting position of the drag.