perf(map): optimize geoPathsFor method

[marcelgerber]

Okay, well, I must admit I got sidetracked when working on map annotations ;)

I noticed the way we first generate a SVG path for a country, then tear it apart to round numbers to one decimal place, and then reconstructing the SVG path is not very performant.

It turns out that d3 allows passing a context to the projection functions. The context is normally used to render to, say, a canvas element, but it's also perfectly fine for our use case here.

And indeed, in my very unscientific testing, geoPathsFor takes:

• 30-40ms before this optimization
• 12-15ms now

The return value is totally unchanged, which the SVG tester will confirm.

[marcelgerber]

With the latest commit, there are now expected non-visual SVG diffs:
We now use Math.round(x * 10) / 10 rather than the previous x.toFixed(1). This makes it so a L538.0,288.9 becomes L538,288.9, for example (i.e. we drop the unnecessary trailing zero).

Also, once we get the chance to update to d3-geo@3.1.0, we can use its now-inbuilt feature instead (d3/d3-geo#272).