Introduce fast paths to speed up NULL color conversion somewhat, particularly when...

Introduce fast paths to speed up NULL color conversion somewhat, particularly when using 64-bit code;  on the decompression side, the "slow path" also now use an approach similar to that of the compression side (with the component loop outside of the column loop rather than inside.)  This is faster when using 32-bit code.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.4.x@1521 632fc199-4ca6-4c93-a231-07263d6284db