Support resources in subdirectories in the Drag-And-Drop interface

[javagl]

When dragging-and-dropping a .gltf file that refers to a resource (texture or buffer) that is contained in a subdirectory, together with that subdirectory, into the Drag-And-Drop interface, the validator complains about missing resources.

Common viewers (Like https://gltf-viewer.donmccurdy.com/ or https://sandbox.babylonjs.com/ ) are able to resolve these resources when they are dropped together with the .gltf file (and they run the validator on them, and are not causing errors in this case...)

I think the validator should be able to resolve these resources as well.

As an example, the "Duck" asset, modified to put the texture and buffer into subdirectories.

Duck with subdirectories.zip

[lexaknyazev]

Duplicate of #232.