Needed for test files that have smaller/larger sizes

- scene node list must only contain root nodes
- nodes must not have multiple parents
- cycle detection

Used to be an assert and consequently untested

The MSFT_texture_dds in the test files is there so other importers (CgltfImporter, really) can share test files. Also for CgltfImporter, the invalid basisu index test needs to be separate because that's checked by cgltf at import.

and mention their existence in the docs

The accessor checks for these were previously untested

Originally moved out for CgltfImporter, but it doesn't perform these tests in openData() anymore, so they don't need to be in separate files

std::isdigit requires <cctype>, apparently MSVC 2019 transitively included it somewhere already

Not allowed, and used to lead to an assert in vertexFormat()

It's used in other places already, be consistent and allow easier search for integer parsing