GLTF: Add import_pre_generate and export_post_convert extension steps

[aaronfranke]

This PR adds 2 missing hooks to GLTFDocumentExtension:

• import_pre_generate: Runs at the start of scene generation, before generate_scene_node
• export_post_convert: Runs at the end of node conversion, after convert_scene_node

Each of these can be worked around in most practical situations by using import_post_parse instead of import_pre_generate, and export_preserialize instead of export_post_convert. However, depending on the specifics of the use case, this could result in several problems. It could result in the data being potentially inconsistent and incorrect between calls to append_from_file and generate_scene, or between append_from_scene and write_to_filesystem, respectively. import_pre_generate is placed after the skins/skeletons, so if the code depends on those, it couldn't go in import_post_parse. Additionally, the lack of these methods may cause missing steps on re-runs of code, or pointless re-running of code.

All that said, I am making this PR with a use case in mind. I need export_post_convert to properly support a new system for glTF joints. I need to run some calculations at the end of node conversion, but I also need to know the indices of all nodes, so convert_scene_node is too early, and to ensure the data is correct at all stages, export_preserialize is too late. I have tested this PR on the aforementioned joints project (plus a test of import_pre_generate) and it works correctly.

[fire]

There's some conflicting lines.

[akien-mga]

Thanks!