Do not load textures via model-path

[rdb]

Textures are currently loaded along model-path, and panda3d-gltf permanently prepends the glTF directory to the model-path in order to find the texture. This has undesirable side-effects, such as described here: https://discourse.panda3d.org/t/pandas-caching-system-has-many-issues/28995

Instead, the glTF converter should do what Panda's egg loader does:

• Resolve the texture itself, relative to the model, passing the full path to load_texture()
• Set the original, relative filename on the returned texture using set_filename()
• Stop modifying the user's model-path.

It would need to be tested that this change works with all texture conversion modes.