Continued preecho tuning/fixes. Gone to average dB with even/odd

Continued preecho tuning/fixes.  Gone to average dB with even/odd
smoothing, and cranked the sensitivity back up to ala rc3.  Result:
when rc3 and current are working 'properly', current trips _slightly_
more impulse blocks but also catches many more audible cases.  Current
also seems more resistent to false positives.

svn path=/trunk/vorbis/; revision=3181