Warp pointer on first/last pixel

Previously, the pointer would get warped when it had left the window.
This doesn't work when sxiv is run in fullscreen mode (or similar -- see
dwm's monocle mode): The pointer can never leave the window in such
situations.

To fix this, warp the pointer when it's on the first/last pixel. To
avoid an endless warping loop, the new position must be at least one
pixel away from the next warp position.