Based on 1/2*log2(N)-19/8, but with the 2-bit and 3-bit thresholds
shifted by 2*log2(N)/8 bit and log2(N)/8 bit, respectively.

to be cleared at the end of the state structs.

This simplifications also fixes a few bugs in the PLC (hope it didn't create some new ones).

Fixed a few minor bugs in the process.

No change to behaviour except that for stereo we now only force a minimum
of one energy bit per channel (previously two because of an error).