Transparency ordering

[ProfLander]

• Currently, transparency ordering is only correct versus other instances with the
  same mesh within the same batch
  • This is down to sorting each batch into contiguous blocks of a given mesh,
    then sorting the instances in those blocks by depth
• It's fundamentally more performant due to scaling by O(mesh_count) calls,
  but also fundamentally incorrect w.r.t. transparency ordering
• Correctness requires that depth supercedes mesh for sorting purposes
  • Batches could be sorted by depth, and then a series of indirect draw calls
    could be generated for each contiguous block of a given mesh
• However, is this worth it given that transparency ordering cannot take place across different batches?
  • With this context, order-independence is the only truly correct solution
  • Intuition says favour performance for now and document technicalities around instanced draw order
    • Mesh / Material key types can control ordering at the type level for user control
    • Only affects transparent objects
    • Extra complexity not worth it for correctness within the same material, but incorrectness vs others