GLTF: Support MultiMesh objects using EXT_mesh_gpu_instancing

[aaronfranke]

Fixes issue #109280, supersedes PR #107866.

Draft because this depends on PR #113202, PR #113206, PR #113245, and PR #113247.

This PR adds support for importing glTF EXT_mesh_gpu_instancing nodes as Godot MultiMesh nodes:

In master, this extension was not supported, which meant that such objects were not imported as multiple instances.

This PR also adds support for exporting MultiMesh nodes as glTF EXT_mesh_gpu_instancing, and configuring the export behavior. Users can choose to use the extension, use plain glTF nodes, or use both:

Note that the dropdown only appears when the scene contains at least one MultiMeshInstance3D node, so it won't clutter the export settings UI in most cases where the user does not have any such nodes in their scene.

The options have the following behavior:

• Optional EXT_mesh_gpu_instancing: Convert MultiMesh nodes into the EXT_mesh_gpu_instancing extension. The extension is marked as not required, so implementations without support can still load the file, but they will see a single mesh instead of multiple instances of meshes.
• Required EXT_mesh_gpu_instancing: Convert MultiMesh nodes into the EXT_mesh_gpu_instancing extension. The extension is marked as required, so implementations without support cannot load the file.
• Multiple Nodes Only: Convert MultiMesh nodes into multiple glTF nodes. This is similar to the existing behavior in master, but there are multiple bugs with the current master, which have been fixed in this PR.
• Multiple Nodes Fallback: Convert MultiMesh nodes into both the EXT_mesh_gpu_instancing extension and multiple nodes, allowing it to be imported as a MultiMesh when supported, or imported as nodes when not supported.

Here's what it looks like when loading a glTF exported with "Multiple Nodes Fallback" into Godot 4.5-stable:

This PR also makes several related changes:

• Adjust SceneImportSettingsDialog::_fill_scene to handle MultiMeshInstance3D nodes.
• Add a new function _export_configure_for_scene to GLTFDocumentExtension, allowing extensions to be configured before the export process starts, so that they can customize what properties are shown.
• Unify Mesh to ImporterMesh conversion logic and move it to ImporterMesh::from_mesh.
• Add a function to GLTFState to generate a unique name (previously was private to GLTFDocument).
• Fix a bug where MultiMesh's own transform was multiplied into each instance.
• Fix a bug where 2D MultiMesh transforms were swapping the Y coordinate into Z.
• Fix a bug where the material name was used as the surface name instead of the surface name.
• Fix an issue where extensions added export settings before the base glTF settings finished being added.
• Fix an issue where buffers and accessors were not loaded by the time nodes were parsed. Nodes can depend on buffers and accessors, so the buffers and accessors need to be loaded first.
• Fix an issue where buffers and accessors were serialized before nodes were serialized. Exported nodes can write data to buffers and accessors, so the buffers and accessors need to be serialized after.

[fire]

Adding multimesh support to godot engine gltf is a good idea. Need to pick a time to review. Other reviewers are welcome too.

[aaronfranke]

I split this into PR #113202, PR #113206, PR #113245, and PR #113247. These can be merged first, and then I can rebase and undraft this PR.

[fire]

Seems to be blocked on #113247