Handle partition type coding of boundary blocks

The partition types of blocks sitting on the frame boundary are
constrained by the block size and the position of each sub-block
relative to the frame. Hence we use truncated probability models
to handle the coding of such information.

100 frames run:
yt 0.138%

Change-Id: I85d9b45665c15280069c0234ea6f778af586d87d
7 files changed