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