We now never process mirroring or mipmaping with multithreading
it's just not worth it given the overhead of the jobsystem.
We also require 64 lines per job, below that, we only multithread per
face (6 threads). This should probably depends on the sample count,
but we don't have this facility yet.
Special case downsampling for mipmaping.
This alone improves performance by 2x for small cubemaps (e.g. 16x16).
