Added first cut at progressive jpeg loading. Currently does not handle

Added first cut at progressive jpeg loading. Currently does not handle

999-10-29  Michael Fulbright  <drmike@redhat.com>

        * src/io-jpeg.c: Added first cut at progressive jpeg loading.
        Currently does not handle either progressive jpeg files (jpeg
        files with multiple scans at different quality settings), but
        I plan on adding this support soon. These are fairly rare in
        my experience, so it shouldn't slow people down too much.
        Grayscale jpegs also don't work but that should be easy to fix.