vc1: Factorize out chroma MC
vc1: Add avg_no_rnd_vc1_chroma_mc4_c()

Needed for proper interlaced support.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
vc1: Enable the interlaced B-frame codepath.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>