I write an exporter, which exports PBR materials, mostly from the PrincipledBSDF shader. Would like to add a small directory of generic environment textures in my zip file, in a format that Blender does not use. Like a 2 umbrella studio environment texture, seen here as a reflection texture.
User could then also pick from a custom property list, in addition to a the .hdr in the world node. The file would get copied to the export directory.
Problem is how do I know where the directory from the addon is, in a completely OS independent way?