add osm3d map type

[kfarr]

things to do before making this available

• - better handle 3 dependencies of osm-tiles, osm-geojson, and osm2geojson, right now they're all stuffed into osm-tiles-geojson.js (now using webpack on this fork: https://github.com/3DStreet/osm4vr/)
• buildings and maps should not be selectable / clickable
• - better handle removing buildings when switching maps
• - (!) do NOT save all the tiles generated by osm3d as children in 3dstreet json, instead ensure that any children of street-geo are not saved; look at example from environment component
• - better handle copyright notices adding / hiding for OpenStreetMap
• - test saving / loading

[kfarr]

wip:

[kfarr]

things this doesn't support:

• this uses the default tile.openstreetmap.org service. The osm4vr component does not yet support proper appending of api key in tile image requests to use other, paid hosted tile services.

[vincentfretin]

The code looks sane. I have no specific comments on this one.