Hide game window when advancing a dead player to the next level

Commit 88b5e616a9 ("Replace calls to
window_set_visible in DoPlayerDead() with stop/start_time()") switched
from hiding the game window to only stopping time, and that only if
hiding the window would have stopped time.  In multiplayer, this allows
time to continue running.  This introduced a crash if the player dies
during the mine countdown.  When the player dies, the game checks if the
reactor has been destroyed.  If so, the game immediately advances to the
next level.  That advance will try to draw the game window if it is
visible.  When the window is drawn, if time is not stopped, then game
logic runs.  Some of that logic is not prepared to deal with the
inconsistent state present when a new level is only partially loaded.
That inconsistent state then causes a crash.  Call stack:

    #11 slew_frame () at similar/main/slew.cpp:152
    #12 in d2x::object_move_one () at similar/main/object.cpp:1758
    #13 in d2x::object_move_all () at similar/main/object.cpp:1956
    #14 in d2x::GameProcessFrame () at similar/main/game.cpp:1848
    #15 d2x::game_handler () at similar/main/game.cpp:1615
    #16 in dcx::window_send_event () at common/include/window.h:116
    #17 dcx::event_process () at common/arch/sdl/event.cpp:176
    #18 in newmenu_do2 () at similar/main/newmenu.cpp:498
    #19 in newmenu_do2<dcx::unused_newmenu_userdata_t> () at common/main/newmenu.h:184
    #20 newmenu_do<dcx::unused_newmenu_userdata_t const> () at common/main/newmenu.h:190
    #21 newmenu_do<1ul, dcx::unused_newmenu_userdata_t const> () at common/main/newmenu.h:196
    #22 net_udp_wait_for_requests () at similar/main/net_udp.cpp:4563
    #23 net_udp_level_sync () at similar/main/net_udp.cpp:4607
    #24 in multi_level_sync () at similar/main/multi.cpp:3458
    #25 in d2x::StartNewLevelSub () at similar/main/gameseq.cpp:1803
    #26 in d2x::StartNewLevel () at similar/main/gameseq.cpp:2018
    #27 in d2x::AdvanceLevel () at similar/main/gameseq.cpp:1648
    #28 in d2x::DoPlayerDead () at similar/main/gameseq.cpp:1721

The root cause of this is the layering violation:
- Killing the player can have the side effect of advancing the level
- Advancing the level can have the side effect of calling multiplayer code while the level data is in an inconsistent state
- Calling multiplayer code can cause the event system to redraw the game
- Redrawing the game can cause game logic to run

Hack around this by restoring the logic that hides the game window, so
that the window is not redrawn and the game logic is not run.  This does
not fix the layering problem, but prevents crashing affected users.  To
avoid undoing the feature from the breaking commit, hide the window only
when advancing to a new level, rather than unconditionally.  A player
advancing to a new level already lacks the move-at-cold-start capability
even on successfully escaping the mine, so no functionality is lost with
this change.  Players who are dead and do not advance to a new level
retain that capability.

Fixes: 88b5e616a9 ("Replace calls to window_set_visible in DoPlayerDead() with stop/start_time()")
Reported-by: Ninjared <https://forum.dxx-rebirth.com/showthread.php?tid=1097>

similar/main/gameseq.cpp

@@ -1717,10 +1717,14 @@ window_event_result DoPlayerDead()
 #endif
                 {
 
+			if (Game_wind)
+				window_set_visible(Game_wind, 0);
 			result = AdvanceLevel(0);			//if finished, go on to next level
 
 			init_player_stats_new_ship(Player_num);
 			last_drawn_cockpit = -1;
+			if (Game_wind)
+				window_set_visible(Game_wind, 1);
 		}
 #if defined(DXX_BUILD_DESCENT_II)
 	} else if (Current_level_num < 0) {