x11: Add a hint to force override-redirect.

Fixes #3776.

include/SDL_hints.h

@@ -1659,6 +1659,17 @@ extern "C" {
  */
 #define SDL_HINT_PREFERRED_LOCALES "SDL_PREFERRED_LOCALES"
 
+/**
+ *  \brief Mark X11 windows as override-redirect.
+ *
+ *  If set, this _might_ increase framerate at the expense of the desktop
+ *  not working as expected. Override-redirect windows aren't noticed by the
+ *  window manager at all.
+ *
+ *  You should probably only use this for fullscreen windows, and you probably
+ *  shouldn't even use it for that. But it's here if you want to try!
+ */
+#define SDL_HINT_X11_FORCE_OVERRIDE_REDIRECT "SDL_X11_FORCE_OVERRIDE_REDIRECT"
 
 /**
  *  \brief  An enumeration of hint priorities

src/video/x11/SDL_x11window.c

@@ -372,6 +372,7 @@ X11_CreateWindow(_THIS, SDL_Window * window)
     SDL_VideoData *data = (SDL_VideoData *) _this->driverdata;
     SDL_DisplayData *displaydata =
         (SDL_DisplayData *) SDL_GetDisplayForWindow(window)->driverdata;
+    const SDL_bool force_override_redirect = SDL_GetHintBoolean(SDL_HINT_X11_FORCE_OVERRIDE_REDIRECT, SDL_FALSE);
     SDL_WindowData *windowdata;
     Display *display = data->display;
     int screen = displaydata->screen;
@@ -444,7 +445,7 @@ X11_CreateWindow(_THIS, SDL_Window * window)
         depth = displaydata->depth;
     }
 
-    xattr.override_redirect = ((window->flags & SDL_WINDOW_TOOLTIP) || (window->flags & SDL_WINDOW_POPUP_MENU)) ? True : False;
+    xattr.override_redirect = ((window->flags & SDL_WINDOW_TOOLTIP) || (window->flags & SDL_WINDOW_POPUP_MENU) || force_override_redirect) ? True : False;
     xattr.backing_store = NotUseful;
     xattr.background_pixmap = None;
     xattr.border_pixel = 0;