Misc changes to the GLTF module before audio PR

[aaronfranke]

This PR includes a few changes I made while working on my upcoming GLTF audio PR, but without any audio stuff.

• Fully document GLTFBufferView.
• Partially document GLTFAccessor.
• Improve some documentation in GLTFDocumentExtension.
• Add a method to GLTFState to append a PackedByteArray to the buffers / bufferViews, with optional deduplication.
• Add a method to GLTFBufferView to read the data from this bufferView as a PackedByteArray.
• Allow the node JSON to be empty when calling import_node on extensions. This is required to handle the case of Godot scene -> in-memory GLTF data -> Godot scene, so like a half-export then half-import.
• Allow the scene node to be null when calling export_node on extensions. This is required to handle the case of GLTF file -> in-memory GLTF data -> GLTF file, so like a half-import then half-export. This is an actual use case I have that needs to be supported, and anyway, this is good for code cleanliness to ensure the steps are fully separated.

[fire]

Can you write an example how to use handle the case of GLTF file -> in-memory GLTF data -> GLTF file, so like a half-import then half-export.

I think I know what you mean, but what do you mean? It would help the general godot developer if we were able to put a sample here.

Otherwise I didn't see any obvious problems, but need a testplan -- like project examples of the use cases you mentioned.

[aaronfranke]

@fire Here is an example use case from The Mirror, packing a GLTF into a GLB:

## Converts a GLTF document (including all its external dependencies) to a GLB byte array.
## GLTF files can have dependencies on other files. This would make distribution very complex.
## In order to resolve this, we can convert the GLTF to GLB data at runtime to pack it into one file.
static func convert_gltf_files_to_glb_data(path: String) -> PackedByteArray:
	var state: GLTFState = GLTFState.new()
	var doc: GLTFDocument = GLTFDocument.new()
	var err = doc.append_from_file(path, state)
	if err:
		push_error(str(err))
		return PackedByteArray()
	return doc.generate_buffer(state)

[akien-mga]

Thanks!