New b-intra mode where direction is contextual

Preliminary patch on a new 4x4 intra mode B_CONTEXT_PRED where the
dominant direction from the context is used to encode. Various decoder
changes are needed to support decoding of B_CONTEXT_PRED in conjunction
with hybrid transforms since the scan order and tokenization depends on
the actual direction of prediction obtained from the context. Currently
the traditional directional modes are used in conjunction with the
B_CONTEXT_PRED, which also seems to provide the best results.

The gains are small - in the 0.1% range.

Change-Id: I5a7ea80b5218f42a9c0dfb42d3f79a68c7f0cdc2