[meshlet] use seamless lod for each instance as Nanite-style

[teassy000]

currently, the meshlet's lod follows the mesh, meaning each mesh instance has its own lod. I would like to generate lod for each meshlet in the runtime. This causes the seams and makes the material discontinue.

here's some resource that could help:

Improve support for cluster simplification
zeux/meshoptimizer#704

Cluster-based rendering, Nanite-style (200M triangles in 1ms)
https://www.youtube.com/watch?v=7JEHPvSGaX8

Multiresolution structures for interactive visualization of very large 3D datasets
https://d-nb.info/997062789/34