VCFilter can go unstable

[Dan Ellis]

The variable filter can still go unstable for certain settings. For instance, if we extend the TestFilter case by a few 100 ms by sending the note-off at 900ms instead of 500ms, e.g.

    amy.send(time=0, osc=0, wave=amy.SAW_DOWN, filter_type=amy.FILTER_LPF, resonance=8.0, bp0_target=amy.TARGET_FILTER_FREQ, filter_freq=5000, bp0='0,1,500,0.01,100,0.0')
    amy.send(time=100, note=48, vel=1.0)
    amy.send(time=900, vel=0)

.. then the filter blows up:

We can probably work harder to prevent AMY from trying to design filters with very low center frequencies combined with high resonances.