Ponder LFO EG sustain behavior on modulation

[Andreya-Autumn]

As of 1.2, EG sustain params behave in two ways:

LFOEG's can not have sustain modulated during the sustain phase. Modulation will be in effect during decay, as the decay approaches the moving target, but once sustain phase begins the value is held as discussed here: surge-synthesizer/surge-rack#858

FEG and AEG can have sustain modulated, also once the sustain phase begins. The modulation is slew-limited by the decay rate parameter though. EvilDragon points out that this is probably congruent with many analog designs (in which decay may quite literally be a slew limiter on sustain value), but I don't know if this is expected behavior in a digital synth like Surge.

Changes here would be breaking obviously. So for XT2:

A: Decide on what behavior we would really want.
B: Make sure that behavior is consistent across AEG/FEG/LFOEG