Support for CesiumVectorOverlays::VectorTilesRasterOverlay

[azrogers]

Depends on CesiumGS/cesium-native#1365. This PR adds support for the new VectorTilesRasterOverlay to allow vector 3D Tiles tilesets to be draped over other tilesets. It also in the process adds support for displaying points from GeoJSON documents. Because the vector tiles PR moved the GeoJSON overlay to the new CesiumVectorOverlays namespace, this PR also applies that change here.

[j9liu]

Thanks @azrogers for the PR! I still have to finish the cesium-native PR review and run some test data through this, but just wanted to leave the few small comments I had so far.

[j9liu]

Something else I found... the Unreal Editor unfortunately gets stuck on quit when a vector overlay is in the scene because IsReadyForFinishDestroy never resolves to true. It seems like the async destruction event is never called, maybe something is stuck or not properly destroyed?

[azrogers]

@j9liu Looks like the problem was I set the destructor of VectorTilesRasterOverlay to default when it's the destructor that resolves the promise... should be fixed now. Also, had to fix the CI that broke due to the linux image changing.

[j9liu]

Thanks @azrogers!

I'm just doing a fresh build to verify that the destructor is working properly, but if all goes well, this should be good to merge.