Move gltf to core.

[fire]

See godot-extended-libraries/godot-fire#3.

From reduz:

We got quite a lot of demand to make the GLTF importer part of core, given it is needed for VR and other stuff, so I have the feeling we should optimize it for performance (which we should do anyway) and have an option to load actual meshes instead of EditorImporterMesh.. so that means we would need to provide a function to parse the mesh node into a scene node and the editor supplies, otherwise it uses Mesh as a default

General design:

var gstate : GLTFState = GLTFState.new()
var gltf : PackedSceneGLTF = PackedSceneGLTF.new()
print(path);
var raw_f = File.new()
if raw_f.open(path, File.READ) != OK:
	return FAILED
var buffer : PackedByteArray = raw_f.get_buffer(raw_f.get_len())
var root_node : Node = gltf.import_gltf_scene_buffer(buffer, flags, 1000.0, gstate)

Todo from reviewers:

• add explicit security checks and migrate any URI resolving code for textures & buffers & etc. into a common helper which has an explicit check that returns ERR_FAIL_...
• EditorSceneImporterMeshNode to MeshInstance needs to be function pointers that allow the code to switch based on function pointer null to MeshInstance3D
• Blend shape key frames should have service limits requested in Gltf export blend shape #48947
• Review design of PackedSceneGLTF vs purely using GLTFDocument

[fire]

Superseded by: godotengine/godot-proposals#3273