Support polygon heightReference and perPositionHeight

[ggetz]

heightReference is currently only supported when height is defined and won't work if perPositionHeight: true.

Requested: https://groups.google.com/forum/#!searchin/cesium-dev/jessica$20gettings%7Csort:date/cesium-dev/nPBzPKQhqqg/QnKZgvsXAgAJ

[hpinkos]

heightReference is not currently supported with perPositionHeight. I submitted a PR to add warning messages: #6978

I'm going to reword this issue for adding support for this

[lyaunzbe]

Any movement/timeline on this issue by chance? Thanks you