Filter balance behavior is borked

[mkruselj]

Currently, filter balance parameter works in a very very unexpected way compared to other synths that have this feature.

Namely, it does not only mix between the filter outputs themselves, it also mixes what comes into the filters in the same way. So, for example, if we route oscillator 1 into both filters, and we use Serial 1 configuration, the expectation is that filter balance would mix between output of filter 1 (which is just osc 1 into filter 2) and output of filter 2 (which would in this configuration be osc 1 into filter 1, then into waveshaper, then into filter 2, summed with dry osc 1 again).

However, what we actually get is that as you move filter balance from F1 to F2, osc 1 input into filters 1 and 2 is also mixed, so with filter balance at max (filter 2 only), we ONLY hear osc 1 going into filter 2, without any prior processing by filter 1 and the waveshaper there. Highly uncommon!

Proposed solution for this would be adding a RMB parameter on Filter balance parameter that would disable this coupling between filter input and output mixing, and only do mixing of outputs. Paul said that this is pretty tricky because a new argument needs to be threaded into QuadFilterChain, and do some (a decent amount of) SSE to make it happen. So let's mark this one as a potential candidate for 1.9!

[mkruselj]

This issue is a subset of #4355, so linking to it before closing this one.