x11: check if the X server honored our XMoveWindow() call (thanks, R.E. Rust!). This can happen if a window is still grabbed when we try to move it, or if the X11 ecosystem is just in a bad mood, I guess. This makes sure that SDL will report the correct position for a window; otherwise, SDL_GetWindowPosition will just report whatever the last SDL_SetWindowPosition call requested, even if the window didn't actually move. Fixes Bugzilla #4646.