Fixed crash when writing statically-typed glTF extensions that aren't registered

[lilleyse]

I'm using some statically-typed glTF extensions that are custom to my application. Previously the code assumed that if an extension wasn't registered it must be a JsonValue, which would cause a crash due to a bad any_cast. Now statically-typed extensions that haven't been registered are ignored.

[kring]

How do you feel about raising a warning when the extension isn't registered, instead of silently ignoring it? I ask because I've run into the crash before, and actually found it useful ("oh yeah, I forgot to remove that extension"). I think it's just a matter of adding a method to JsonWriter to report (and store) warnings, calling it from writeJsonExtensions when createExtensionHandler returns nullptr, and then copying those warnings to e.g., TilesetWriterResult in TilesetWriter::writeTileset. The reader side has a similar warning mechanism already.

[lilleyse]

I think that's a good idea. I'll try that out it a bit.

[lilleyse]

@kring this is ready for another look

[kring]

Thanks @lilleyse!