When updating default_partition_cdf, this sums the probabilities that
were divided evenly across the pairs PARTITION_HORZ_A/PARTITION_HORZ_B
and PARTITION_VERT_A/PARTITION_VERT_B. Those summed probabilities now
get distributed evenly across the triples you get by adding
PARTITION_HORZ_4 and PARTITION_VERT_4, respectively.

Rather than implement 2X8/8X2 blocks for now, ss_size_lookup returns
4X8/8X4 block sizes to use as chroma transform sizes for 4X16/16X4
blocks.

The changes in setup_pred_plane and set_skip_context are because this
is presumably the first time we've had to deal with 16x4 or 4x16
blocks. Since BLOCK_16X4 is not less than BLOCK_8X8, the existing
logic didn't work (and the "shuffle back one" logic should probably be
done for small widths and heights separately).

Change-Id: If28d8954da42d6c726f2bcce2cb5242154b0870c