Blender Git Loki

Blender Internal: optimization to remove sleep() calls from the render threading
code. This gives a speedup up to a couple of seconds based only on the image
resolution and tile size. For complex renders a second or two is not so noticeable
but for quick ones it's nice to save some time. On the default cube this gives me
about half a second speedup.

Patch by Johan Walles, based on the render branch implementation by me.