Added update of mode context pointers in decoder

With the NEWENTROPY experiment enabled encoding certain clips
produced invlid bitstreams, or files that had a high degree
of artefacts.

This was the results of pointers in MACROBLOCKD not being
setup correctly (mode_info_context and prev_mode_info_context).

Change-Id: Ice13e1efa8bd122997d2f8f3f1e761c6c16e0403