Use the Assets plugin for handling RenderMesh creation

[RiscadoA]

Problem

Currently, the render mesh plugin is a bit (very) cursed because it performs reference counting and garbage collecting on the created render meshes. This is the kind of task that the assets plugin was created to solve. We shouldn't duplicate its logic.

Solution

We could make the render meshes assets.
E.g., RenderMesh would be an asset, loaded by a RenderMeshBridge, associated to the .mesh format, or something like that.
The .mesh.meta would specify the original source which produces this mesh, which would be used to generate the mesh, instead of loading it from a .mesh file - i.e., the .mesh file doesn't exist in this case, only the .meta.

We should make sure that it is still easy to procedurally generate grids and create render meshes from them. I think that with this change it would be very weird, we might need to add some method to Assets to create a new asset with a given .meta and call the corresponding bridge, or something like that 🤔