audioconvert: add optional hilbert transform to rear channels

Add an option to do a hilbert transform on the generated rear channels
to do a 90 degree pahse shift on them. This can improve spacialization
of the rear channels.

See #861

src/daemon/client-rt.conf.in

@@ -87,4 +87,5 @@ stream.properties = {
     #channelmix.lfe-cutoff = 0
     #channelmix.rear-delay = 12.0
     #channelmix.stereo-widen = 0.0
+    #channelmix.hilbert-taps = 0
 }