* libopna: Fixed channel 3 extend mode

* libopna: Now bit-perfect with actual OPNA/OPN3 chip under limited conditions, including stereo output (glitch?) when ALG >= 4
  (Envelope is not perfect, only perfect when AR >= 21)
example tone (extracted from th9_flrn.ovi/th9_fl9z.ozi):
@  0 5 6
 31  0  0  7  0  18 0  4 3 0
 16  1  0  8  1   0 2  4 3 0
 15  1  0  8  1   5 0  8 1 0
 16  1  0  8  1   5 2  4 0 0

Known issues:
* FM envelope is not very accurate (example: yuno_27.m86 track 3 tone 10)
* PMD SSG envelope is very buggy