Reworked context conversion between 8x8 and 4x4

Reworked context conversion between 8x8 and 4x4

The commit rationized and simplified the entropy context conversion
betwen MB using 8x8 transform and MB using 4x4 transform. The old version
had a number of weirdness in how 4x4 transform MB's context is used for
8x8 blocks other than the first 8x8 within a MB.

Test showed the change has a gain ~.1% for avg psnr, glb psnr and ssim on
the limited HD set.

Change-Id: I774536c416baa6845aa741f956d8a69fa40e5d47