Limit number of glTF validation issues

[javagl]

Addresses #290

glTF tile content is validated with the glTF-Validator. When validating a glTF asset that contains many errors, this can lead to an out-of-memory error. (For example, the GLB in the linked issue would generate roughly 6 million issues...).

The glTF validator offers an option to limit the number of issues that are reported. It's hard to pick a "universally sensible" value for that.

(One critical corner case could be that when the limit is set to x, then there may be an asset that generates x warnings or infos, and the x+1th issue would be an 'error', leading to the asset counting as 'valid' even though it isn't...)

This PR just adds a fixed maxIssues: 1000 for now, which prevents the issue. Questions beyond that are:

• How many issues should be the default?
• Should this be configurable from the 3d-tiles-validator side (via validation options in a config file)?
  • (If yes, some thought would have to go into that. It could be a maxContentIssues for all types of content, but only the glTF Validator currently supports such a 'limit'...)

[javagl]

There is now an upper limit for the number of issues that are reported by the glTF-Validator.

This limit is 1000 for now, but this not specified. The point is: When a glTF asset generates 1000 issues, then this is a single issue, namely, 'the glTF asset being 'bad''.

If necessary, that limit could be made configurable (via the validator config JSON file). But it's unlikely that an end-user would like to change this value, say, from 1000 to 1200. If necessary, it can always be made configurable later.