no strokeDasharray on markers

[Fil]

closes #2131

[mbostock]

What about the alternative fix of putting the marker into a defs element (or simply outside the g element, somewheres) so that it doesn’t inherit attributes? Would that be a more general fix? For example, is there a similar issue with the fill and stroke-opacity being inherited (or is that inheritance desirable)?

(Was wondering if there would be an issue here if strokeDasharray were specified as a channel, but that’s not supported, so it’s nice that we can simplify. Was also wondering if we should always set stroke-dasharray="none" but I guess it’s nice to avoid if necessary.)

[Fil]

a) I believe stroke-opacity is useful to inherit. fill doesn't really make sense in any of the supported marks.

b) I researched the idea of using an external <defs> group, but I wouldn't want to add that group to the top-level svg (it makes it harder to control multiple renderings with, say, a pointer transform: the number of defs might grow unbounded). We could nest the current <g> under a wrapper <g> that also includes <defs> as a sibling of the styled <g>, but it feels "too much" given point a) above.

[Fil]

Side note: in the arrow mark, which has potentially a fill and a marker, the marker has the same fill color … and that looks ok.

[mbostock]

I simplified this to just always set stroke-dasharray=none, which I feel is more consistent with how we handle the other attributes. That causes some redundant effort (especially since stroke-dasharray is rare), but it seems fine.