Support downmix to mono in Multitrack Song Export #2099
Description
All tracks are exported as stereo with no option for mono. Drum shells especially would be mic'd Mono in a studio, stereo tracks for every kit piece are overly complex and not well suited to panned mixing in a DAW. I have to run a SoX batch command on exports to make them mono. An export option for this would be very useful.
(see #2096 for discussion)