Fix issue with placement in per-monitor configuration.

When two monitors are on different desks, ensure that the correct
monitor is used when placing windows. Otherwise placement policies
can use the wrong desk when computing where to place a window.
This is done two fold, first initialize windows with a NULL monitor
so current monitor is used in cases a monitor is not specified
for the window to start on, and second delay updating the window's
monitor until after the window has been placed, so the correct
monitor is found based on the windows location.

This fixes #1043.

fvwm/add_window.c

@@ -205,8 +205,12 @@ static void setup_window_structure(
 	FW_W(*pfw) = w;
 	if (savewin != NULL)
 	{
+		/* JS: 2024-10-19
+		 * Set initial monitor to NULL, so current monitor can
+		 * be used for placement and finding current desk.
+		 */
+		(*pfw)->m = NULL;
 		(*pfw)->Desk = savewin->Desk;
-		(*pfw)->m = savewin->m;
 		SET_SHADED(*pfw, IS_SHADED(savewin));
 		SET_USED_TITLE_DIR_FOR_SHADING(
 			*pfw, USED_TITLE_DIR_FOR_SHADING(savewin));
@@ -2333,8 +2337,6 @@ FvwmWindow *AddWindow(
 	{
 		rectangle attr_g;
 
-		update_fvwm_monitor(fw);
-
 		if (IS_SHADED(fw))
 		{
 			state_args.do_shade = 1;