Return fringe bitmaps of row containing position POS in window WINDOW.
If WINDOW is nil, use selected window. If POS is nil, use value of point
in that window. Return value is a list (LEFT RIGHT OV), where LEFT
is the symbol for the bitmap in the left fringe (or nil if no bitmap),
RIGHT is similar for the right fringe, and OV is non-nil if there is an
overlay arrow in the left fringe.
Return nil if POS is not visible in WINDOW.
(fn &optional POS WINDOW)