Switch datasources to use polylines on terrain

[likangning93]

At least KMLDataSource is using corridors for polylines clamped to terrain, but now we can use polylines when available.

What other datasources can be updated?

[hpinkos]

@likangning93 GeoJsonDataSource takes a clampToGround argument and is currently using a corridor.

[mramato]

These should be be trivial changes, so it would be nice to have them in the next release.

[likangning93]

Does this include adding clampToGround to Czml polylines?

[mramato]

Does this include adding clampToGround to Czml polylines?

Yes, does the entity API default to true or false?

[likangning93]

It defaults to false. I'm not sure about what else needs to be edited though in terms of CZML documentation/wiki stuff.

[mramato]

@likangning93 the czml-writer repository is where all of that stuff gets handled. CzmlDataSource itself will likely be a one line change. You can work with @shunter to get the property added to the spec.

Glad to hear it defaults to false, since that makes it a non-breaking change.