Interleave modes/residual per macroblock

Packs the bitstream with each mb's residual following its mode/mv
information.

TODO: There are still a few fields that should be packed into partition
0 but are included in partition 1, due to them being serialized from
write_kfmodes/pack_inter_mode_mvs, which execute after the first
partition is finalized. These need to be separated out into a separate
function, similar to mb_mode_mv_init() in decodemv.c.

Change-Id: I43a46c363601ab36954d07ebe498760e1e2e3af4