Mix2 is busted

[nshaheed]

TriOsc t => Pan2 p => Mix2 m => dac;

-1 => p.pan;

1::week => now;

The following code does not work for RC1 and does not work for 1.4.2.0. It's still hard-panned when outputted to the dac.

Probably what needs to happen is to have an actual tick function for mix2. Also it would be nice to have a Mix2.mix method that sets the balance of the two inputs. Right it's just a Gain ugen that doesn't even work. Alternatively, we could deprecate it.