Consider filtering out obsolete infos from glTF validator

[javagl]

The 3D Tiles Validator currently supports the validation of several glTF extensions. Under the hood, this is implemented by

• using the glTF Validator to perform the baseline validation
• dive into the glTF to perform the extension-specific validation

Calling the glTF-Validator with a glTF that contains an unknown extension will still cause an 'INFO' message saying

"Cannot validate an extension as it is not supported by the validator: '...'."

These messages could be fitered out by default for the extensions that are validated by the 3D Tiles Validator. On this level, this could be pretty simple.

A slightly more challenging aspect of this could be to also filter out the messages that refer to potentially unused elements. When a certain texture or accessor or attribute is only used in an extension, then the glTF Validator will report

                  "type": "CONTENT_VALIDATION_INFO",
                  "path": "/textures/3",
                  "message": "This object may be unused.",
                  "severity": "INFO"

The goal would then be to filter out this message if and only if the object is used in one of the known extensions. This is technically possible, by looking at the path and matching this against the elements in the glTF that have been found to be used in the extension. But transporting that information back from the validation process and applying it as a filter to the output that is generated by the glTF-Validator may require some thought.