textures=ref can leave absolute Windows paths in gltf output

[izzyboris]

Related to Moguri/panda3d-gltf#131. The glTF spec only addresses relative uris for image paths, leaving any additional uri handling up to implementors to deal with. blend2bam should default to using relative paths where possible.

On Windows at least, converting in 'ref' mode can leave absolute paths in the outputs where the uri probably should be relative to the model asset.