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.
Updated unstable version number
Fixed image orientation if a JFIF APP0 segment is present in a JPEG header.
Seek over EXIF APP0 frame instead of reading and discarding it
Fixed inconsistencies in navigation when removing invalid files