[WIP] Transparency sorting

[kvark]

Closes #182
cc @eddyb

This is just a start. The plan is to have some abstraction for passes, and then split the instances (not batches!) between the transparent/opaque passes based on the is_opaque result. The passes are going to be sorted by the distance to camera (in separate orders). Instancing is handled differently:

• for opaque pass, we aggressively form large batches of instances, averaging the center of the batch between the instances, and only then sorting.
• for transparent pass, we don't do any batching. As an improvement, we could form some instanced batches after sorting, guaranteeing that the order will persist.