Struct holding geometry optimised per SubMesh / LOD level, ready for copying to instances.
Since we're going to be duplicating geometry lots of times, it's far more important that we don't have redundant vertex data. If a SubMesh uses shared geometry, or we're looking at a lower LOD, not all the vertices are being referenced by faces on that submesh. Therefore to duplicate them, potentially hundreds or even thousands of times, would be extremely wasteful. Therefore, if a SubMesh at a given LOD has wastage, we create an optimised version of it's geometry which is ready for copying with no wastage.