MECmpContext.ildct_cmp is an array 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().
Remove these pointers from MECmpContext and add pointers
for the actually used functions to its users. (The DV encoder
already did so.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>