Improve GLTF export logic for scene root nodes

[aaronfranke]

• Fix GLTFDocument::_convert_scene_node writing data into the JSON array for the scene root node. At this stage of the export process, it is not appropriate to touch the JSON yet, in fact the data that was being written here was being ignored and overwritten. Instead, it now writes the root node to p_state->root_nodes.
• Improve GLTFDocument::_serialize_scenes to no longer be hard-coded for a single root node. Now it just uses whatever data was given to it in p_state->root_nodes.
• Improve GLTFDocument::_serialize_scenes to have the failure condition at the top with a message, instead of having it check if (p_state->nodes.size()) { and return invalid data if false.
• Move the code to add a buffer above export_preflight and convert_scene_node so that the buffer is available to be used in those steps.
• Fix an inconsistency of r_state vs p_state in the headers vs the cpp (I used r_state, let me know if p_state is preferred instead and I can change it to be consistently p_state).

This PR should not change behavior except that p_state->root_nodes and the buffer are now populated during export, and there will not be data prematurely written to JSON and overwritten later.

[fire]

Looks ok

[akien-mga]

Thanks!