-Fix some bugs in row_scan and col_scan. In some cases, the above
or left neighbor was not considered even though it is available.

-When above or left neighbor is not available, try using the
top-left, top-right or bottom-left neighbor.

Compression improvement:
lowres   0.20%
midres   0.16%
hdres    0.20%

Change-Id: If521665589c7f29277b8e9223f21f4a8bf3fef39