Another fix for an issue affecting the encoder on macOS #28

See:

• Use max 16 threads Unity-Technologies/crunch#14 by @blaztinn

The commit 4bb735f7966eedf414aa8e24533967cffd5bbcdb only fixed the threading limit on the decoder part. Use the limit also in the encoder.

Without this change we got random black squares on converted textures when compressed on Macos with 24 threads.

The issue was not reproducible on Windows but I've added the limit there also in case it is a race issue that is harder to trigger on Windows.

@ArctypeZach you may also be interested in testing that so we can merge it.