x11: add a hint to force the VisualID used when creating a window.

include/SDL_hints.h

@@ -197,6 +197,12 @@ extern "C" {
  */
 #define SDL_HINT_VIDEO_X11_XRANDR           "SDL_VIDEO_X11_XRANDR"
 
+/**
+ *  \brief  A variable forcing the visual ID chosen for new X11 windows
+ *
+ */
+#define SDL_HINT_VIDEO_X11_WINDOW_VISUALID      "SDL_VIDEO_X11_WINDOW_VISUALID"
+
 /**
  *  \brief  A variable controlling whether the X11 _NET_WM_PING protocol should be supported.
  *