MECmpContext has several arrays of function pointers that
are not set by ff_me_cmp_init(), but that are set by users
to one of the other arrays via ff_set_cmp().
One of these other users is mpegvideo_enc; it is the only user
of MECmpContext.frame_skip_cmp and it only uses one of these
function pointers at all.
This commit therefore moves this function pointer to MpegEncContext;
and removes the array from MECmpContext.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>