send & return: mutually inclusive

[andr-ew]

• if a track is set to both the send & return, it will not send to itself, only send to other tracks and receive from other tracks
• alternative: when on both the send & return busses, pre_level is set to 0 and old sets outgoing level_cut_cut levels (not routing audio into itself). this lets two tracks feed back into each other, ping-pong style
• the default behavior for send & return could involve mutual exclusivity, but tapping both keys at once could enter into a special send + return mode, which sets things up nicely for multi-track feedback loops

[andr-ew]

with stereo tracks coming into the fold we can stick w/ the first option