x11: Ungrab before attempting a confinement grab with XInput2 enabled

If XInput2 is enabled, it will grab the pointer on button presses, which results in XGrabPointer returning AlreadyGrabbed. Clear any existing grabs before attempting the confinement grab to avoid a timeout scenario.

src/video/x11/SDL_x11window.c

@@ -1855,6 +1855,14 @@ int X11_SetWindowMouseGrab(SDL_VideoDevice *_this, SDL_Window *window, SDL_bool
             return 0;
         }
 
+        /* If XInput2 is enabled, it will grab the pointer on button presses,
+         * which results in XGrabPointer returning AlreadyGrabbed. Clear any
+         * existing grabs before attempting the confinement grab.
+         */
+        if (data->xinput2_mouse_enabled) {
+            X11_XUngrabPointer(display, CurrentTime);
+        }
+
         /* Try to grab the mouse */
         if (!data->videodata->broken_pointer_grab) {
             const unsigned int mask = ButtonPressMask | ButtonReleaseMask | PointerMotionMask | FocusChangeMask;