skins2(Linux): WM_CLASS also set for the invisible root window

(cherry picked from commit 9aa3dfaefa8cadf870856cf89318073fd911a42e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/skins2/x11/x11_display.cpp

@@ -209,6 +209,12 @@ X11Display::X11Display( intf_thread_t *pIntf ): SkinObject( pIntf ),
         m_mainWindow = XCreateWindow( m_pDisplay, root, 0, 0, 1, 1, 0, 0,
                                       InputOutput, CopyFromParent, 0, &attr );
 
+        // initialize WM_CLASS
+        XClassHint classhint;
+        classhint.res_name = (char*) "vlc";
+        classhint.res_class = (char*) "Vlc";
+        XSetClassHint( m_pDisplay, m_mainWindow, &classhint );
+
         // Receive WM_DELETE_WINDOW
         Atom wm_delete = XInternAtom( m_pDisplay, "WM_DELETE_WINDOW", False);
         XSetWMProtocols( m_pDisplay, m_mainWindow, &wm_delete, 1);