Fixes issues with stereo saving and dynalloc

1) Stereo saving was being too aggressive because it only considered the LF
   and because the savings were multiplied by coded_bins rather than by
   just the number of "side bins" below the intensity threshold
2) In the case of bandlimited signals, dynalloc would allocate way too many
   bits to the last non-zero band. We now explicitly check for the last
   band with a meaningful signal.

These issues were really obvious when encoding the decoded test01.mp3
because of the strong inter-channel correlation and (especially) the 16 kHz
lowpass used by the mp3 encoder.