x11: Try to keep SDL_WINDOW_FULLSCREEN* in sync with window manager.

So if Gnome/KDE/etc have a keyboard shortcut or titlebar decoration to
make any window go fullscreen (with the _NET_WM_FULLSCREEN flag on the
_NET_WM_STATE property), we update the SDL window flag.

Fixes libsdl-org#5390.

src/video/x11/SDL_x11events.c

@@ -1434,6 +1434,13 @@ X11_DispatchEvent(_THIS, XEvent *xevent)
                     }
                 }
 
+                if (changed & SDL_WINDOW_FULLSCREEN_DESKTOP) {
+                    /* FULLSCREEN_DESKTOP encompasses two bits: SDL_WINDOW_FULLSCREEN, plus a bit to note it's FULLSCREEN_DESKTOP */
+                    const Uint32 fsmasked = flags & SDL_WINDOW_FULLSCREEN_DESKTOP;
+                    data->window->flags &= ~SDL_WINDOW_FULLSCREEN_DESKTOP;
+                    data->window->flags |= fsmasked;
+                }
+
                 if (changed & SDL_WINDOW_MAXIMIZED) {
                     if (flags & SDL_WINDOW_MAXIMIZED) {
                         SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_MAXIMIZED, 0, 0);

src/video/x11/SDL_x11window.c

@@ -222,7 +222,7 @@ X11_GetNetWMState(_THIS, Window xwindow)
         }
 
         if (fullscreen == 1) {
-            flags |= SDL_WINDOW_FULLSCREEN;
+            flags |= SDL_WINDOW_FULLSCREEN;  /* !!! FIXME: arguably this is more like FULLSCREEN_DESKTOP...? */
         }
 
         /* If the window is unmapped, numItems will be zero and _NET_WM_STATE_HIDDEN