x11: Set the skip taskbar and skip pager atoms on utility windows

Some compositors apparently still need these set to keep the window out of the taskbar and pager, even with the utility window atom being set.

src/video/x11/SDL_x11window.c

@@ -121,6 +121,8 @@ void X11_SetNetWMState(SDL_VideoDevice *_this, Window xwindow, Uint32 flags)
     Atom _NET_WM_STATE_MAXIMIZED_HORZ = videodata->_NET_WM_STATE_MAXIMIZED_HORZ;
     Atom _NET_WM_STATE_FULLSCREEN = videodata->_NET_WM_STATE_FULLSCREEN;
     Atom _NET_WM_STATE_ABOVE = videodata->_NET_WM_STATE_ABOVE;
+    Atom _NET_WM_STATE_SKIP_TASKBAR = videodata->_NET_WM_STATE_SKIP_TASKBAR;
+    Atom _NET_WM_STATE_SKIP_PAGER = videodata->_NET_WM_STATE_SKIP_PAGER;
     Atom atoms[16];
     int count = 0;
 
@@ -136,6 +138,10 @@ void X11_SetNetWMState(SDL_VideoDevice *_this, Window xwindow, Uint32 flags)
     if (flags & SDL_WINDOW_ALWAYS_ON_TOP) {
         atoms[count++] = _NET_WM_STATE_ABOVE;
     }
+    if (flags & SDL_WINDOW_UTILITY) {
+        atoms[count++] = _NET_WM_STATE_SKIP_TASKBAR;
+        atoms[count++] = _NET_WM_STATE_SKIP_PAGER;
+    }
     if (flags & SDL_WINDOW_INPUT_FOCUS) {
         atoms[count++] = _NET_WM_STATE_FOCUSED;
     }