Scaletempo maintains the audio pitch when playback rate != 1.0 (i.e.
no chipmunk effect).  This fixes the pitch scaling caused by using the
resampler to handle playback rate.

Ported from GStreamer.  Inspired by SoundTouch library by Olli Parviainen.
Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>