Fix possible bug when a per-segment deblocking filter level value
needs to be set in non-absolute mode, i.e. when the loop filter update
value is negative in delta mode.
Also clamp the resulting filter level value to 0..63 range.
Improve condition to disable the loop filter. The previous heuristic
used to check all filter levels, for all segments. It turns out that
only the base filter_level value defined in the frame header needs
to be checked.
This fixes 00-comprehensive-013.