The multiple mixing setting has been removed. Multiple mixing is always done and dipping is avoided for adjacent track entries.
This is pretty close to complete. Mixing `a -> b` where both key property `x` avoids dipping. If the mix is interrupted by `c`, the dipping is properly mixed out. However, if `c` *also* keys `x`, a dip is seen.
This is good test JSON data:
http://n4te.com/x/1948-6b1G.txt
The problem can be seen by doing `m1 -> m1-dup` then interrupting the mix with `m1`.
Related issues: #621, #815, #899, #900
