bug#71763: [PATCH] Inconsistency in bitmap overlay drawing for macOS

[Eli Zaretskii <eliz@gnu.org>]

> From: Daniel Pettersson <daniel@dpettersson.net>
> Date: Tue, 25 Jun 2024 00:04:27 +0200
> 
> On macOS overlay-arrow is not draw on top of bitmaps already present
> in the fringe, like its done on X Window System (src/xterm.c).  This is
> to the detriment of gdb-mi.el where the overlay-arrow hides the
> breakpoint when they exist on the same row.
> 
> It can be reproduced with the following:
> (require 'gdb-mi)
> (defvar overlay-arrow (make-marker))
> (add-to-list 'overlay-arrow-variable-list 'overlay-arrow)
> (setq fringe-indicator-alist '((overlay-arrow . hollow-right-triangle)))
> (overlay-put (make-overlay (pos-bol) (1+ (pos-bol))) 'before-string
>              (propertize " " 'display
>                          `(left-fringe breakpoint warning)))
> (move-marker overlay-arrow (pos-bol))
> 
> I expect that macOS is not using overlay_p param as intended as fixed by
> attached patch.  But I was having an hard time finding any documentation
> proving my point other then the implementation of it's X Window System
> sibling in src/xterm.c.

Can some macOS expert please review the proposed patch?