Use platform-specific path separator

[BlueSkia]

When importing a gltf, for textures the addon simply joins the path that exists in the material custom properties. This path uses Windows-style path separators (\), which on Linux and Mac OS causes the file load to fail because it produces a path that looks something like /home/user/meddle-export/cache/modded\abc\def\ghi\file.tex.png. This basically results in a broken import where the user has to manually apply every texture to every relevant material and node.

This patch uses bpy.path.native_pathsep to convert these paths to whatever is appropriate. gltf imports then just work fine and textures are loaded properly.

(I'm super new to blender and python scripting so whatever I'm doing wrong or in a weird or inefficient way please point it out).

[PassiveModding]

looks good to me and verified still working on windows, honestly I expected path.join to have fixed that already but seems it was a misunderstanding on my end.
Thanks