[NOT READY] GeoJSON support

[incanus]

This has some issues, but is a good first cut. It follows very closely the design on the WebGL side, which pre-tiles GeoJSON features after fetch/parse using multiple Transform objects, one per zoom level supported, so that tiles never need to be recalculated.

Things that need work:

• Rendering is awaiting the universal clip_id work so that partial overlays can happen.
• Once render works, support zooms { 1, 5, 9, 13 } as on the WebGL side.
• Possibly rethink the multiple Transform approach.
• Un-hardcode references to the route.js that we've been using like here and here.
• Possibly leverage TileParser for bucket creation instead of doing it manually.
• Round out other feature types besides just LineString.
• Support FeatureCollection.
• Finish rejoin.
• Possibly break out other source types into separate classes with a Source abstract superclass. This will reduce raster-related and other branching.

[incanus]

Ok, putting this on hold for a while in light of other priorities. Just merged master and we have a couple build errors in GeoJSONSource::tile_GeoJSON around direct access to buckets for parsing (see comment about TileParser in original ticket description). Main thing we need here is a parallel parsing routine that can create buckets, but otherwise this is done for basic parsing and rendering of LineString features.