Improve mesh LOD system workflow

[Lasuch69]

Describe the project you are working on

A Small scene aiming for high fidelity with small amount of dynamic objects.

Describe the problem or limitation you are having in your project

LODs are great but you need to keep scenes for just a mesh to have them reliably stored as sometimes you loose them as a local mesh instance which is annoying as i want to have only mesh from imported files so that i can have everything consistent.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Make LODs on the geometry instance basis with a inspector menu which would allow for setting custom LODs or use the generated ones. This change would make automatic LOD system straight forward, more powerful and would remove a lot of inconvienience with assets workflow.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

This would look similar to the "Geometry" section.

"LOD bias" would be moved to a new section called "LOD" and from there you could set custom LOD or just check the "use auto LOD" option or something like that with an option to set how much levels there will be and maybe some parameters for the distance.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No.

Is there a reason why this should be core and not an add-on in the asset library?

Auto-LOD and Godot's UI is in core.

[Calinou]

Related to #5174.

LODs are great but you need to keep scenes for just a mesh to have them reliably stored as sometimes you loose them as a local mesh instance which is annoying as i want to have only mesh from imported files so that i can have everything consistent.

Mesh LOD generation could be supported for OBJ mesh import (or meshes saved from a scene in Advanced Import Settings), but it's not trivial. In the meantime, you can configure OBJ files to be imported as scenes in the Import dock instead of meshes.

[Lasuch69]

It would be nice if you could just use "array mesh" resource for storing in files rather than whole scenes to have whole functionality of the engine as you can't (as stated above) use LOD on this resource in itself :(

[A-Lamia]

I'm not sure if it was talked about but it would be nice if you could select what LOD levels are manual and what are auto in my case i wouldn't mined being able to set how many LOD levels and what ones are auto vs manual.