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 the motion estimation API.
It uses MECmpContext.(me_pre|me|me_sub|mb)_cmp. It is
basically the only user of these arrays.

This commit therefore moves these arrays to MotionEstContext;
this has the additional advantage of making motion_est.c
more independent from MpegEncContext.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>