fader activation for echo effect

[Be]

The Pioneer DJM 909 hardware mixer has a neat feature on its echo effect. When lowering a volume fader to let the post-fader echo effect play its echo buffer, the output of the effect is activated by lowering the fader. The DJ does not need to think about manipulating the dry/wet knob, they only need to lower the fader. It would be useful to add this as a binary button parameter for the echo effect. Implementing this will require making the state of the channel fader accessible as a feature on the GroupFeatureState passed to the effect.

Here is a video demonstrating the effect:
https://www.youtube.com/watch?v=352bAB_Pjgg

[Be (be.ing)]

Related: Bug #1370837

[jus (jus)]

If implemented, paves way for lp:661917

[Be (be.ing)]

I started working on this and realized it would be pretty complicated to implement well. If the Echo effect does not output when the fader is up, there is a sudden jump when raising the fader to the top. I'm not sure how to work around that.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/8895