More compute_allocation() fixes.

1) Continue to update left and percoeff if we skip all the way to the
    first band.
   This doesn't actually matter for correctness, but I don't want to
    try to explain in a spec why we aren't doing this.
2) Force all the bits in skipped bands to go to fine energy.
   Before some of them could continue to be given to pulses, even though no
    pulses would actually be allocate for them.