Don't "downmix" mono to stereo

- This fixes a playback issue with video files that have mono
  audio streams. This was causing it to copy mono audio data
  as if it was stereo which would be filled halfway with garbage
  causing nasty pulsing noise in the output and a slowed-down
  audio track (playback was still mono, only data was copied
  as if it was stereo).
Fix playlist rowheight

- The tableView:rowHeight: delegate method is supposed to
  return a CGFloat which when building 64-bit is a double
  and not a float. This caused the rows to have zero height.