meta-monitor-manager-xrandr.c: DeleteMonitor before SetMonitor (#693)

A somewhat similar fix appears in mutter [1].

The RandR protocol [2] states concerning RRSetMonitor that:

> If 'name' matches an existing Monitor on the screen, the
> existing one will be deleted as if RRDeleteMonitor were called.

Unfortunately, this behavior is not the case in practice, and if an
existing monitor with the same name exists the server generates a
BadValue error [3].  While this was fixed very recently in the Xorg
xserver [3], it is unclear when these changes will make it to most
distributions due to how long it has been since a formal Xorg xserver
major release.  Therefore, we must ourselves prevent muffin from
aborting if a monitor with the same name already exists, a common
condition upon restarting cinnamon or calling `cinnamon --replace`.

The mutter fix [1] solves the problem of preventing mutter from aborting
on the error, but the request still fails to set the monitor's outputs.
The mutter fix also uses an API which is not in muffin yet.

Therefore, we take the approach of using xcb to first delete any monitor
with the name of the one which we wish to set.  Note that this request
can also fail if no such monitor exists, so we explicitly ignore this
error.

A word of warning to those who may wish to use xcb to also replace the
SetMonitor call:  a crashing bug was only recently fixed in libxcb [4]
which caused all calls to xcb_randr_set_monitor() to crash or otherwise
cause the calling program to behave in an undefined manner.  While the
fix is released in libxcb 1.17, this version is not even available in
Ubuntu 24.04, for instance.  Therefore, it may be some time before we
can reliably call xcb_randr_set_monitor().

[1] https://gitlab.gnome.org/GNOME/mutter/-/commit/8d3696f39a0b3af725b7615f7e2ac74ce5e0bcbf
[2] https://cgit.freedesktop.org/xorg/proto/randrproto/tree/randrproto.txt
[3] https://gitlab.freedesktop.org/xorg/xserver/-/commit/146bb9b2c19fb75b7629b65d5871969b7fcef97a
[4] https://gitlab.freedesktop.org/xorg/lib/libxcb/-/commit/038636786ad1914f3daf3503ae9611f40dffbb8f

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -1082,6 +1082,8 @@ meta_monitor_manager_xrandr_tiled_monitor_added (MetaMonitorManager *manager,
   GList *outputs;
   GList *l;
   int i;
+  xcb_connection_t *xcb_conn;
+  xcb_void_cookie_t cookie;
 
   if (!(meta_monitor_manager_get_capabilities (manager) &
         META_MONITOR_MANAGER_CAPABILITY_TILING))
@@ -1117,6 +1119,11 @@ meta_monitor_manager_xrandr_tiled_monitor_added (MetaMonitorManager *manager,
       xrandr_monitor_info->outputs[i] = output->winsys_id;
     }
 
+  xcb_conn = XGetXCBConnection (manager_xrandr->xdisplay);
+  cookie = xcb_randr_delete_monitor_checked (xcb_conn,
+                                             DefaultRootWindow (manager_xrandr->xdisplay),
+                                             name_atom);
+  free (xcb_request_check (xcb_conn, cookie)); /* ignore DeleteMonitor errors */
   XRRSetMonitor (manager_xrandr->xdisplay,
                  DefaultRootWindow (manager_xrandr->xdisplay),
                  xrandr_monitor_info);