libvpx

a number of fixes to entropy stats collection

1. block types
There are only three types of blocks for 8x8 transformed MBs, i.e. Y
block with DC does not exist for 8x8 transformed MBs as all MB using
8x8 transform have 2nd order haar transform. This commit introduced
a new macro BLOCK_TYPES_8X8 to reflect such fact.

2. context counters
This commit also fixed the mixed of context_counters between 4x4 and
8x8 transformed MBs. The mixed use of the counters leads me to think
the existing the context probabilities were not properly generated
from 8x8 transformed MBs.

3. redundant collecting in recoding
The commit also corrected the code that accumulates entropy stats by
making sure stats only collected for final packing, not during the
recode loop

Change-Id: I029f09f8f60bd0c3240cc392ff5c6d05435e322c