A new block mode, referred to as skip_mode is added. If a block is
coded as skip_mode, it will be inter-coded, with its references and
motion vectors derived from its neighboring blocks with zero-residue.
Otherwise, the block can be coded in the current intra or inter mode.

The computational load on skip_mode evaluation at the encoder should
be kept minimum. No transform size / type evaluations are needed.

Change-Id: I5aef0159c7d5ecd64258510835903375d6c536d6