Fix SDL_SetWindowPosition on fullscreen windows.

This reverts http://hg.libsdl.org/SDL/rev/7cdeb64faa72 and fixes it in
the correct way. If you call SDL_SetWindowPosition on a fullscreen
window, it would update the x & y variables for the window, but not
actually move the window (since it was fullscreen). That would make the
internal state of the SDL_Window incorrect, causing
SDL_WarpMouseInWindow to offset incorrectly.

This makes it so SDL_SetWindowPosition updates the `windowed' x & y
coordinates, which take effect when you revert from fullscreen.