fixed gltf export when using paths instead of only file name

[dabeschte]

When you store a GLTF file using a path instead of only a file name, this line effectively duplicated the path.

E.g., instead of writing all files into out when using scene.export("out/scene.gltf"), it saved all files in out except for the gltf file which is stored in out/out/mesh.gltf - raising an exception doing so.

[mikedh]

Thanks for the fix!! What's the combination of export args that hits this? I was trying to test with variants on this but couldn't quite get it:

    from tempfile import TemporaryDirectory
    with TemporaryDirectory() as d:
	trimesh.creation.box().export(d, file_type='gltf')
        r = trimesh.load(os.path.join(d, 'model.gltf'))

[dabeschte]

I can produce the error with this script (assuming that "fused" folder exists):

import trimesh
m = trimesh.load("example.glb", force="mesh")
m.export("fused/example.gltf")

error message:
[Errno 2] No such file or directory: C:\\Users\\dabeschte\\Desktop\\test_meshes\\fused\\fused/pbr_cubes_emissive_fused.gltf

the same happens when I use double backslashes or os.path.join