Allow editing arbitrary pre-existing meshes

[Zireael07]

Describe the bug
I added a mesh to the parent mesh instance, then added the ply editor. Ply editor continutes to behave as though the mesh was empty.

To Reproduce
See above

Expected behavior
Expected ply editor to allow me to edit the preexisting mesh.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

[jarneson]

This is definitely something we've looked at but it's a bit tricky. Godot Meshes expose tris only, so you end up with a triangulated mesh.

We also have stricter constraints on what a mesh is as well, in that it has to be a manifold. Not all Godot Meshes will be manifolds, so determining what to do in that case is tricky.

There's been some work on trying to get this to work in this branch: https://github.com/jarneson/godot-ply/tree/load-mesh-on-creation

[Zireael07]

Yeah yeah, having used the plugin a bit I get the limitation, BUT adding the node underneath some existing mesh and seeing nothing happen is definitely weird.

For now, maybe add a print or a popup somewhere that explains clearly to the end user that any existing meshes will be ignored and overwritten by whatever ply editor spits out?

[fire]

I have a proposal.

If we can determine the pre-existing mesh is manifold we should be able to load it. Let's separate loading from make manifold.

Problems:

1. loading
2. make manifold

[fire]

We're now able to load manifold geometry meshes.

[jarneson]

Let's keep this issue around for "Arbitrary Meshes", which is a very long term goal.

#49 covers Manifold meshes only, which will be solved by the time we have a stable Godot 4 implementation.