x11: Don't update grab on enter when the mouse is captured

The xserver will still send EnterNotify events while the pointer is captured, and the grab shouldn't be updated in these cases, as it will cause the capture to be lost.

(cherry picked from commit 33e5f4885a)

src/video/x11/SDL_x11events.c

@@ -1338,8 +1338,10 @@ static void X11_DispatchEvent(SDL_VideoDevice *_this, XEvent *xevent)
             SDL_SendMouseMotion(0, data->window, SDL_GLOBAL_MOUSE_ID, false, (float)xevent->xcrossing.x, (float)xevent->xcrossing.y);
         }
 
-        // We ungrab in LeaveNotify, so we may need to grab again here
-        SDL_UpdateWindowGrab(data->window);
+        // We ungrab in LeaveNotify, so we may need to grab again here, but not if captured, as the capture can be lost.
+        if (!(data->window->flags & SDL_WINDOW_MOUSE_CAPTURE)) {
+            SDL_UpdateWindowGrab(data->window);
+        }
 
         X11_ProcessHitTest(_this, data, mouse->last_x, mouse->last_y, true);
     } break;