NMFMorph: Sort out assign fiasco

[weefuzzy]

The whole business with having to cycle autoassign in NMFMorph is silly and bad, so it's time to fix it. There's actually two aspects to this:

1. Most urgent: we keep and fix the current behaviour so that autoassign actually does what it says. We can do this, I think, by change tracking the pointers on the input buffers (we don't need to deref them, just compare stored with current). This should tell the client if a buffer has changed in some drastic way, and can be used to set an 'needsAssign' flag to pass through. If the pointer hasn't changed, but the buffer content has, then a toggle would still be needed.
2. Get that whole business out of the audio thread. That's a much more complex proposition, but really the assignment calculation needs to happen in response to a message and then there needs to be some sort of copy-update to pass the new assignment to the audio thread. That entails a sort of hybrid object we don't yet have a model for.