Use rel for CesiumCartographicPolygon

[lilleyse]

Based on discussion in #632 there a few disadvantages of CesiumCartographicPolygon being a subclass of BasisCurves. We should consider using rel instead.

[r-veenstra]

Decisions from standup

• Prefered UX approach is to keep a single CesiumCartographicPolygonprim, vs splitting into two
• @corybarr will look further into isolating the issue
• If we can't figure it out prior to release, we'll likely update CesiumPolygonRasterOverlay to support pointing directly to BasisCurve + Globe Anchor combo, rather than the relapproach to keep UX simple
• CesiumPolygonRasterOverlay can be re-added at a later time if needed

[corybarr]

• @corybarr will look further into isolating the issue

Having created a minimal example that does touch our codebase, I don't think we can keep a single CesiumCartographicPolygon prim for this release.