Style on X-Ray mode

[defvol]

To be able to inspect dense tilesets

Reference: https://www.mapbox.com/blog/turbo-xray-mode

[aaronlidman]

• fwiw here's how studio does this with xrayify.js, search for it
• and here's how old studio-classic maps were styled: https://github.com/mapbox/tilelive-vector/blob/master/templates/layer.xml