#210 [fill between]: computation issues if soft clip and plot domain is the same

[Christian Feuersänger]

https://tex.stackexchange.com/q/449829/95441 reveals a problem with soft clipping.

The code complaints about
Package pgf Warning: fill between skipped: the soft-clipped first input path is empty. on input line 31.

Debugging into the code reveals that the first point of the first path is on the soft-clip path (as expected). There is some special treatment of this case which leads to a wrong decision, namely that the entire path is not inside of the clip path. I suppose the wrong decision was introduced in order to solve another issue -- but it fails.

Suggested solution: run a further computation in order to compute the decision "is it inside or not"