Use Mapbox GL for overlap maps

[planemad]

Overlap maps looks really useful to add context to mapwarper maps or other external maps. Moving from raster to GL based maps using Mapbox GL can unlock some new features:

• Add geojson overlay/Insert overlay underneath basemap labels https://www.mapbox.com/mapbox-gl-js/example/geojson-layer-in-stack/
• Extrude polygons for 3d visualization https://www.mapbox.com/mapbox-gl-js/example/3d-buildings/

How to:

• GL Layer switcher https://www.mapbox.com/mapbox-gl-js/example/toggle-layers/
• Adjust raster layer opacity https://www.mapbox.com/mapbox-gl-js/example/adjust-layer-opacity/

[answerquest]

@planemad Looks great, but moving this over to mapbox is beyond my scope for now. (Really need to sit down and learn mapbox some day!) I suggest you start this as another repo, and I'll link to there. Please feel free to copy over code chunks like the function for interpreting URL arguments.