Update spec file to include glTF extension

[javagl]

Fixes #295

The main point of the ValidFeatureIdAttributeWithByteStride.glb file was to test the validation of the EXT_mesh_features feature IDs in a glTF asset that involves a byteStride in the respective buffer view.

(This was mainly motivated by the earlier approach that manually extracted the accessor data, and where taking into account the byte stride caused the usual caveats. Now, the accessor values are read with glTF-Transform, meaning that we don't really care about the byte stride any more, and just rely on glTF-Transform delivering the right data)

The .GLB file was originally created by loading the .gltf file into gltf.report, and exporting it again. But... this did cause the actual EXT_mesh_features extension to be omitted, as pointed out in #295

The updated file here was created programmatically, with the EXTMeshFeatures extension object of the 3D Tiles Tools being registered in the glTF-Transform NodeIO, meaning that the extension information is retained when it is converted to a GLB with a byte stride.