Rename get_surface_material to get_surface_override_material

[clayjohn]

The surface material property of MeshInstance3Ds overrides the Material for a given surface. It behaves the same as override_material just on a per-surface basis. This has been poorly communicated to users who often mix up MeshInstance3D.get_surface_material() with Mesh.surface_get_material(). This becomes especially problematic when users set a material on the mesh and then try to get it from the MeshInstance3D as in #31558

Renaming the property to reflect that it is an override material is consistent with the override_material property of GeometryInstance3D and makes it clear that it is not the same things as a mesh material

Partially implements: godotengine/godot-proposals#2589
Implements: #31558 (comment)

[lawnjelly]

Sounds sensible to me. 👍

[akien-mga]

Thanks!

[lukostello]

while this is a great improvement. I hope you have considered these options:

1. naming the mesh resource surface materials "default_surface_materials" if you want it to be disambiguated from both sides.
2. instead of "surface_override_material" it could be "instance_surface_material" this communicates effectively that it is unique to each instance and future proofs it if we ever switch from an override to an overlay method.
3. The mesh resources surface_get_material seems wierd to me. Seems like it should be get_surface_material (or get_default_surface_material if you use suggestion 1. or even get_default_material(surface_name)

if there is a reason why it should be surface_get_material() then it seems the format of get_surface_material_override should also be surface_get_material_override()