Non-interlacing alpha

[dy]

An observation by @monfera.

The SVG behaviour is preserving alpha on overlapping segments

[dy]

Covered by overlay option. Pattern gets lost on place of intersection, but multiple lines overlap fine.

Not sure we can implement svg overlapping properly since we are going to implement colorscales #19 - it is just not clear how to blend patterns with different half-transparent colors but blending them like we do now, and should we? Increased opacity on intersections intuitively makes more sense, unless we can find counter-cases.

[dy]

@monfera if you have any considerations please give them here) I will close for now.