Encode a block line by line, horizontally or vertically. In the vertical
mode, each row is predicted by the reconsturcted row above;
in the horizontal mode, each column is predicted by the reconstructed
column to the left.

The DPCM modes are enabled automatically for blocks with horizontal or
vertical prediction mode, and 1D transform types (ext-tx).

Change-Id: I133ab6b537fa24a6e314ee1ef1d2fe9bd9d56c13