GLTFExporter: Push nodes parent-first instead of child-first

[aaronfranke]

Related to #31112

Description

Currently, if you have a Three.js Object3D hierarchy like this:

A
  B
  C

Then it will get exported to glTF child-first: node A at index 2, node B at index 0, and node C at index 1. Node A will have the "children" array set to [0, 1] and the glTF scene will have its root nodes set to [2].

With this PR, it will instead export to the glTF parent-first: node A at index 0, node B at index 1, and node C at index 2. Node A will have the "children" array set to [1, 2] and the glTF scene will have its root nodes set to [0].

Any order is fine for the most part, since glTF itself does not explicitly require any order, and does not suggest a preferred order. glTF importers can import any order, and changing the order does not affect compatibility. Blender uses the same order, too. However, UnityGLTF and Godot both use parent-first order. I'd argue it's more human-readable this way, because the first node in the array tells you the start of the tree, and well, if you display it visually like I did above, you put the root at the top anyway. Most importantly, GODOT_single_root has a requirement that the root node is at index 0, since the goal is to make the "scene" and "scenes" properties fully redundant, as they are usually not useful for glTF files representing "one object". Changing the order Three.js exports nodes in matches GODOT_single_root and other game engines, with no downsides, so it should be done regardless of whether or not the exported file has the GODOT_single_root flag in it.

[donmccurdy]

Still thinking about the other PRs, but this one is a clear win/win in my opinion — let's do it, thanks!