Remove Math.log in LZW calculation of next code size.

Remove Math.log in LZW calculation of next code size. The LZW codes always increase one bit at a time.

This code was added in 03c5fa3d19bac01a16245ee727b07477b8cabb2f.  I don't really see a natural log call as a good optimization over a switch statement, but this code can be made much simpler anyway.

I'm not sure if there are any good test cases covering this, I reckon it's okay though : )