Clarify extension usage validation

[javagl]

Currently, the use of extensions is validated in (what I first thought to be) a straightforward way. One aspect of that validation is:

• When there is an extension found during the traversal (for example, a boundingVolume.extensions["3DTILES_bounding_volume_S2"]), then the "3DTILES_bounding_volume_S2" is stored as a "found" extension in the validation context
• At the end of the validation, the "found" extensions are compared to the extensionsUsed that have been declared. When an extension was "found" but not declared as "used", a warning is created. When an extension was declared as "used", but not "found", a warning is created.

The problem (which is obvious in hindsight): Certain extensions are never "found" explicitly. For example, 3DTILES_content_gltf. It never appears as an object anywhere, but only allows glTF/GLB files to be used as tile content.

This can be solved, to some extent, by keeping track of the actual content data types that have been encountered. (There are corner cases where this becomes less trivial. For example, when the content data validation is disabled via a command line flag).

A related question is the version dependency. A specific example is that 3DTILES_content_gltf should probably not be used in a 1.1 tileset, and using it should cause a warning. The more general question is: How to encode the mix-and-match between "tileset versions" and "allowed extensions". There may be extensions that are valid only in 1.0, other may be valid only in 1.1, and others may be valid in both...