Add a new boolean surround-delay property that makes
audioecho just apply a delay to certain channels to create
a surround effect, rather than an echo on all
channels. This is useful when upmixing from stereo - for example.

Add a surround-mask property to control which channels
are considered surround sound channels when adding a
delay with surround-delay = true

Original patch from Jochen Henneberg <jh@henneberg-systemdesign.com>