Add support for surround recordings

[Benjamin Klum]

Right now, recording from inputs offer up to 2 channels. Recording from track offers up to 8 channels. Both should offer up to 16.

[Benjamin Klum]

re #268 Raise worker channels from 8 to 16

→ <<cset eaa35f032d97>>

[Benjamin Klum]

re #268 Correctly detect number of track channels if recording from track input

→ <<cset 8271bc3e949e>>