ARW: Avoid copying to speedup with ARW2 threading
ARW: Avoid copying to speedup with ARW2 threading

Allocate the output buffer at the beginning and give each thread
it's own mutable slice to work with. This required using unsafe
to produce the mutable slices but should be completely safe as
the slices are all non-overlapping.

pedrocr/rawloader,pedrocr/rawloader