Set cors headers for vtiles

[springmeyer]

Companion to #1215.

This is intended to allow testing vtiles produced by Studio locally against mapbox-gl-js styles like:

{
  "sources": {
    "sourcename": {
      "type": "vector",
      "maxzoom": 14,
      "tiles": ["http://localhost:3001/source/{z}/{x}/{y}.vector.pbf?id=tmsource:///path/to/sourcename.tm2source"]
     }
  }
  [ .. snip ... ]
}}

[springmeyer]

@yhahn question: I already see cors() used in the routes for the tile requests (https://github.com/mapbox/mapbox-studio/blob/mb-pages/lib/server.js#L110-L116). Could it be that those are not working as expected or are they not at all related to providing cors headers?

[yhahn]

Hrm yes, maybe a candidate for http request/response testing territory : )

[springmeyer]

Could it be that those are not working as expected or are they not at all related to providing cors headers?

Got to the bottom of this: the cors() middleware works on successful requests but on 404's it never kicks in which makes it look like it is not working while debugging.