Use an int MotionVector for find_block_motion.

Using the double variant causes several pointless conversions between
double and int.
Worse, one of the conversions is in an inner loop together with a
function using MMX, resulting in undefined behaviour.
Based on debugging by Ray Simard.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Tested-by: Ray Simard <rhs.ffmpeg@sylvan-glade.com>