Add GeoBuf support for GeoJSONSource#setData()

[tikhonbelousko]

mapbox-gl-js v0.15.0:

I have a 90k of lines in my GeoJSON and I have a server which encodes it in GeoBuf which results in 1.6 Mb, if I used GeoJSON it would be 14Mb. Now I decode my GeoBuf to JS object and set it using GeoJSONSource#setData() which blocks my UI for ~4s.

As I understand, the fastest way for me to render this stuff is if GeoJSONSource#setData() with provided URL could support GeoBuf data. Which would be blazingly fast, wouldn't it?

[mourner]

If you specify GeoJSON as an url, it will be loaded off the main thread, not blocking anything. Since any browser can accept gzipped resources, loading times shouldn't be a big problem if it's just 14Mb non-gzipped.

Geobuf support may be introduced at some point, but for now, I'll close this as duplicate of #1504.