RobsonFBP

The effect could be more noticeable if the initial volEnvRelease is 1s and I add 6400 to the modulator parameter. However, my intention is not just a vibration effect, but...

### Thanks for this PR. I liked the name of the created variable ` ignore_sutain ` because it is more intuitive than `cancel_default_hold` in your PR I think if `(mod->src1...

I would also like to be able to test each change, but I don't know how to do it. If you could send me a compiler with all the settings...

### What if the **DECAY2** phase was created in the envelope as described in [this commen](https://github.com/FluidSynth/fluidsynth/discussions/1598#discussioncomment-13800769)?

> @davy7125 To achieve this functionality in @davy7125's graph, we need to trigger the release phase whenever the hold pedal state changes, because with a time change, the countdown always...