Investigations into how the fine tuning field affects OPL register writes.

Findings:

 * Fine tuning only affects double voice instruments.  It is only applied to
   the second voice and not the first.

 * The fine tuning value adjusts the index into Doom's frequency table.  The
   offset is divided by two before being applied, so a fine tuning value of
   10 causes an offset of 5.

 * Due to integer rounding, odd numbered negative offsets are rounded down
   (away from zero), so a tuning of -3 gives an offset of -2.