Change trace behavior from min/max to start/length

[DJLevel3]

To make the trace functionality more useful, I propose altering the behavior of the trace effect such that instead of using values for the start and end of the trace, which means you can't loop the trace around, it instead uses values for the start and the length of the trace.

I'll implement this functionality in my fork for now, but the changes should be applicable to the main version before my fork gets merged.

[DJLevel3]

Changes pushed to my fork

[loglot]

I really like this idea in concept, it's painful to keep max above min when using things like the sawtooth wave