[WIP] circle layer

[brunoabinader]

Implements breaking changes of mapbox/mapbox-gl-style-spec#264:

• Circle type

Part of #1740. Follows mapbox/mapbox-gl-js#1192.

[brunoabinader]

As per discussion with @tmcw, we believe it would be a good idea to merge this PR with #1390. However, #1390 seems a bit old so rebasing it against master is quite challenging.

/cc @lbud

[jfirebaugh]

That's correct, this needs to go onto the v8types-merge branch from #1390 (and that branch should be renamed to v8 and the PR rerolled). I'm hopefully going to be working on v8 full time shortly and can look into doing that.

[jfirebaugh]

I'm rebasing this onto an updated v8 branch.