Cosmetic changes in GLTF node generation code

[aaronfranke]

This PR has some internal renames in the GLTF node generation code and some reordering.

• Reorder _generate_scene_node and _generate_skeleton_bone_node arguments to put the node index first. The index is more "core" information to the node generation so it should come first. This may seem slight/pedantic but it was prompted by another change coming after this PR in which it will make more sense to order the arguments this way.
• Fix missing p_ prefix at the start of parameter names in _generate_scene_node.
• Rename _assign_scene_names to _assign_node_names because it assigns node names.
• Rename Ref<GLTFNode> n to gltf_node to improve readability.

[fire]

Some of the code comments were changed to have a p_ underscore, but on general I'm ok with this.

[aaronfranke]

That was on purpose to match the variable names but I can undo it.

[fire]

There's some odd errors? Want to bump the cicd?

Have you been using this in your project? I was going to import the model and spot check https://github.com/KhronosGroup/glTF-Sample-Models but didn't get around to it. Maybe you can assist.

[fire]

Cosmetic

[akien-mga]

Thanks!