*Feature Request* Slew rate on LFO waveform output

[matrix12x]

I was thinking adding slew rate (i.e., glide) to the LFO output, as it modulates a destination, would be cool. It would have zero effect on waveforms like triangle, or sine, but lots of effect on random or pulse/square. It could be a menu parameter.

This is like the feature the sequential Take 5 has. It will enable effectively different LFO waveforms.

We could probably reuse the glide/portamento code to do this feature. (se computeGlide in synth.c)