Return position information for buffer POS in WINDOW.
POS defaults to point in WINDOW; WINDOW defaults to the selected window.
Return nil if position is not visible in window. Otherwise,
the return value is similar to that returned by `event-start' for
a mouse click at the upper left corner of the glyph corresponding
to the given buffer position:
(WINDOW AREA-OR-POS (X . Y) TIMESTAMP OBJECT POS (COL . ROW)
IMAGE (DX . DY) (WIDTH . HEIGHT))
The `posn-' functions access elements of such lists.
(fn &optional POS WINDOW)