Add shield for Western Ring Route in New Zealand

[k-yle]

This PR adds a shield for network=NZ:WRR, which is a one-of-a-kind route (example 1, example 2)

Related to #451

[ZeLonewolf]

Thanks @k-yle for opening a PR.

Looking at this image:

I have questions.

Is the WRR route relation separate from the route 18 relation? I would think that we would want a custom graphic for the route, much like we do for special turnpikes in the United States. This PR declares a pill shield, which draws a circle or pill around the value in ref, and I'm not sure that's really the behavior we're looking for here?

[k-yle]

yes, it's separate from SH 18, (this is the OSM relation). I saw that US:IA uses the same style, and renders similarly to how the WR shields are signposted.

I wasn't sure if a custom graphic was appropriate in this case (?), since it's a pretty much just a circle with text inside.

[ZeLonewolf]

Oh, I see. Black on white WR in a pill does seem like a workable approximation in that case.

[ZeLonewolf]

Code looks essentially correct, needs a tile update to test.

[claysmalley]

I think the plain text approach is reasonable, though it at least seems important that the shield is a circle rather than a pill or oval. If we use ovalShield() and specify a width of 20 it should make the shield circular.

[claysmalley]

Looks good!

[ZeLonewolf]

Thanks @k-yle for your contribution!