Multithread AABB drawing

[scallyw4g]

Because the actual operation of buffering an AABB is quite cheap, we'll need to pack a lot of the operations together. Doing this amortizes the cost of the worker thread starting up the job, which is also cheap, but not free.

We probably want to go with a two-step approach:

1. Start with creating a thread job that just buffers a single AABB
2. Make a job that packs multiple of those jobs togetherj

There's an example of doing this already; grep for work_queue_entry_copy_buffer_set

Alternatively, we could implement instanced drawing, which is probably what we should actually do..