Background recording in Metal

[kvark]

We finally figured out all the unknowns about Dota performance, and the simplified gist of it is that we need to go through recording faster, much faster than we do. Deferred recording allows us to create this illusion of recording very fast, only to delay the actual work to the submission time. The trick here, as indicated in #2232, is the fact the driver and HW are able to do the work at the same time as we chew through the recording.

I was thinking... if there is a way for us to both get to the submission point earlier AND benefit from online command buffer recording. What if we create deferred recording lists but instead of them to wait for the submission time, we start actually recording them into metal command buffers somewhere on a hidden thread? The submission would then just make sure to wait for that thread to finish working on each submitted command buffer before calling commit on it.

What that would give us? Looks like a hacky solution in place, introducing implicit threading, just to squeeze more performance from Dota. It is indeed, but it would be interesting to see if it gives us a solid advantage ;)