Back-port code from jpeg-8 that removes unpopulated (and unneeded) tables for AC...

Back-port code from jpeg-8 that removes unpopulated (and unneeded) tables for AC and DC coefficients when generating progressive JPEG files with arithmetic coding.  This should make such files bitwise compatible with jpeg-8, barring any other mathematical differences (such as the different subsampling algorithm used in jpeg-8.)  Add regression tests for progressive+arithmetic JPEG files.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.2.x@1082 632fc199-4ca6-4c93-a231-07263d6284db