3D geometries limitations with Cesium

[gberaudo]

Cesium has limited support for 3D geometries.

• they can not be draped on the terrain;
• most are always drawn on the ellipsoid, not in 3D.

Related issues are: #4, #11, #12.

Contrary to what was stated in #12, 3D primitive support in Cesium is limited.
From my tests, only the polyline geometry works as expected: given an array
of 3D positions, only the polyline geometries are displayed in 3D by Cesium.
The other geometries (circle, polygon) are displayed on the ellipsoid even
when the positions have an elevation.

A workaround for the case where all the elevations of a geometry are the same,
may be to use a primitive model with a translation value set to the geometry
common elevation. This would however not work with MultiPoint, MultiPolygon
and the GeometryCollection, unless they all share the same common elevation.

[klokan]

Looks like a duplicate ticket to #12 - which is exactly about the limitation of Cesium in this regards - and states that polygons in 3D are not supported at all yet.

This ticket and #12 should be merged to one - and one of them marked as closed probably.

[gberaudo]

Once PR #71 is merged, we can close this issue and follow #12 (draping on terrain).
The PR implements a different trick, compatible with multi-geometries.