audioresample: corrected buffer duration calculation to account for nonzero initial...

audioresample: corrected buffer duration calculation to account for nonzero initial timestamp

Since we calculate timestamps by:

  timestamp = t0 + (out samples) / (out rate)

and durations by:

  duration = ((out samples) + (processed samples)) / (out rate) - timestamp

if t0 is nonzero, this would simplify to

  duration = t0 + (processed samples) / (out rate).

This duration is too large by the amount t0.  We should have done:

  duration = t0 + ((out samples) + (processed samples)) / (out rate) - timestamp

so that

  duration = (processed samples) / (out rate).