Support setting 3.x material/# properties on MeshInstance3D

[nikitalita]

This PR introduces support for setting the old material/# properties on MeshInstance3D, which were renamed to surface_material_override/# in 4.x. This is only supported when using set() (i.e. during the initial load) and emits a warning when doing so.

Note that this should not conflict with ImporterMesh; This will only trigger if the property is named material/{num}. I have checked, and MeshInstance3D was the only resource that stored a property named material INI style in either 3.x or 4.x.

[fire]

If it's not too much hassle can you provide a test sample?

[nikitalita]

Here's an example I threw together: example_meshinstance3d_mesh.zip

Without this patch:

With this patch:

[nikitalita]

@fire same here