Make LoadContext::finish add all dependencies, and avoid a second texture load in GltfLoader.

[andriyDev]

Objective

• The glTF loader previously did two loads for textures: the first did all the expected parsing and loading. The second would intentionally try to perform the same load to A) avoid doing a second load, and B) to add the handle as a dependency specifically for the material - so that the material would not be "loaded with dependencies" until all its textures were loaded.
• In general, it's possible for an asset to have a handle to another asset, without that handle being considered a dependency according to the LoadContext.
• Also, some handles in the StandardMaterial were missing the #[dependency] attribute.

Solution

• Add the dependency attribute to all handles in StandardMaterial.
• Make the gltf anisotropy extension also use the parse_material_extension_texture function like all the other extensions (the existing implementation matched exactly).
• Made LoadContext::finish update the list of dependencies with VisitDependencies.
• Propagate the list of texture handles through the GltfLoader and clone the handle out when necessary.

Testing

• Added tests to bevy_asset to verify tracking dependencies works - this also fails before the rest of this PR.
• Ran the load_gltf example and it still works. This won't show whether the dependency tracking works correctly though.

[ChristopherBiscardi]

read through the code and it all seems reasonable to me. +100 on the deduplication.