gltf28 does not support texture paths outside the blend directory

[Moguri]

Assume the following structure

models
  - model.blend
maps
  - model_base.png

In this case, the blend file will point to ../maps/model_base.png. On export, the Blender 2.80+ glTF exporter will copy this to the same directory as the glTF output file and change the path to model_base.png.

blend2bam should take the output glTF and attempt to fix paths in a post-processing step (similar to how physics is handled).