low pass filter overhaul

[x42]

Currently the Low Pass is a non-standard custom combination of two 2nd order and a low-shelf.
This is mainly to produce a flat 12dB roll-off while maintaining zero-phase at nyquist.

However, the resonance peak widens close to nyquist which impacts the filter steepness and reduces real-world usability to some extend.

[naught101]

Just checking out this plugin - looks pretty awesome. It would be nice to have options for faster roll-off for both the high and low pass. Up to maybe 48dB.