gltf containing absolute (Windows) paths fail to import with duplicated prefixes

[izzyboris]

blend2bam v0.25.0 in 'ref' textures mode can produce gltf outputs containing absolute Windows paths. These break gltf2bam imports by producing a path that has the directory part of the path duplicated. The glTF spec only addresses 'relative' URIs as part of the standard, leaving any more fragile extensions to the uri property of images up to implementors.

It seems reasonable to allow gltf2bam to detect an absolute path and deal with it sanely.

• Using a .blend file referencing an external texture, attempt to base.loader.loadModel("foo.blend") and run the Panda3d program on Windows.
• blend2bam v0.25.0 (with textures=ref) leaves an absolute image reference that panda3d-gltf interprets as, e.g., "/c/Users/username/path/assets/models//c/Users/username/path/assets/models/texture.png"