Inner Box-shadow on pie chart slices

[skoli-elm]

Is your feature request related to a problem? Please describe.
Not a problem really, just a feature update.

Describe the solution you'd like
I wanted to add box-shadow to the pie slices that are currently active. The shadow should be only on the straight edges and not on the curves.

Additional context
Please find the example image below :

[plouc]

I think it's somehow a duplicate of #1488, this might be even harder to achieve, as we would need filters, masks... I don't think that's something the lib can provide by default, it would introduce too much complexity for a very specific effect. #1488 seems more feasible.