Importer add-on support (Import hooks)

[Benjamin-Dobell]

Is your feature request related to a problem? Please describe.
It's currently not possible (without resorting to hacks) to write a companion add-on (implementation of a GLTF extension) which supports round-tripping from Blender to GLTF and back again.

Describe the solution you'd like
A reciprocal glTF2ImportUserExtension API which provides similar, albeit opposite, hooks to glTF2ExportUserExtension.

Describe alternatives you've considered

• Monkey patching.
• Provide a separate import feature which duplicates the GLTF parsing and inserts nodes derived from custom GLTF extensions into the appropriate place within the scene.
• Add the functionality to io_scene_gltf2 directly.

Additional context
I've written a very basic add-on which exports lightmaps from Blender into GLTF, represented by a custom material extension. The lightmap is picked up from the glTF Settings pseudo-group i.e. same workflow as occlusion maps.

I'd like to be able to import lightmaps back into Blender.

[MeFisto94]

I want to bump that, are there any opinions on this?
What work is required? Could this be done relatively simple by starting off from the export extension?

[julienduroure]

I forgot to close this ticket, there is now an import addon extension system