avutil/motion_vector.h: fix coordinate types

See b0352b1997a83f1b6b27919b94aab539f099b25b for more information on the
feature.
doc/APIChanges: fill 2 hashes from my recent API additions
avfilter/vf_lenscorrection: fix memleak

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>