Ext mesh features id clarifications

[javagl]

• The propertyTable of a feature ID should be a glTFid, and not a plain integer ( glTF metadata uses integers where it should use "glTF IDs" 3d-tiles#751 )
• Add clarifications for Clarify details about featureCount and nullFeatureId 3d-tiles#756

This might be fixing CesiumGS/3d-tiles#756 . One aspect of that issue was whether the featureCount is actually required. But there likely are cases where implementors might need this information (so it is unlikely that it will be removed from the specification). And regardless of whether it will be removed or not: As long as it is still contained in the specification, the implementation note should help to clarify its meaning.

[lilleyse]

@javagl could you open a separate PR for the glTFid change? I'm happy to merge that but need to think more about the clarifications for CesiumGS/3d-tiles#756.

[javagl]

New PR at #68

[javagl]

(I think this is obsolete an replaced by #68 now - possible changes for the nullFeatureId and featureCount are independent of this)