Drop shadow improvements

[mthh]

Just a few things about how you define your shadow filter:

• You are adding a useless (as I understand it) feGaussianBlur element, since feGaussianBlur elements don't have to be children of feDropShadow, and since you don't give any attributes to this feGaussianBlur element, you can just remove it, here:

  geoviz/src/effect/shadow.js

  Line 47 in ffe5e5c

  .append("feGaussianBlur");

• feDropShaow isn't supported in Inkscape (see https://gitlab.com/inkscape/inkscape/-/issues/4534), so the whole group (SVG g element) that is linked to this filter isn't displayed in Inkscape (see also Export plante magrit#154 about it and riatelab/magrit@c34519b to see the fix I implemented in Magrit, using feFlood, feGaussianBlur and feOffset instead of feDropShadow).

Also related to drop shadows, I guess you have a small error in the shadow function documentation and could replace blur by shadow here:

geoviz/src/effect/shadow.js

Line 17 in ffe5e5c

* geoviz.effect.shadow(svg, { stdDeviation: 0, id: "blur" }) // where svg is the container