For the first superblock, signal a filter level (6 bit).
For other superblocks, signal the delta (3 bit) and sign (1 bit).
To guarantee the delta can be represented by 3 bits, the search
range of current superblock filter level is capped as
curr_lvl = prev_lvl +- ((1 << 3) - 1)

Change-Id: Ibbe2941aa96ec1220ed2adb7edf3d29cb032f0ec