(overlay_arrow_string_or_property): Remove PBITMAP arg.

(overlay_arrow_string_or_property): Remove PBITMAP arg.
Calls changed.  Don't check for overlay-arrow-bitmap property here.
(overlay_arrow_at_row): Remove PBITMAP arg.  Instead, if left
fringe is present, return Lisp integer for bitmap (or -1 for default).
Fix value of overlay-arrow-bitmap property to be a symbol, use
lookup_fringe_bitmap to parse it.
(display_line): Change call to overlay_arrow_at_row.  Store integer
return value as overlay bitmap in row rather than window.
Only show overlay arrow if row displays text, or if no other overlay
arrow is seen in window (if overlay marker is at point-max).