enable grayscale and luratech modules to handle 16-bit wm_jp2s more gracefully

For a few weeks during April 2007, the addition of watermarks was creating jp2's with 16-bit color depth. Luratech pdf's produced from these images don't get compressed (they end up at ~100KB/page). The grayscale module runs for days without finishing.

Despite how long it's been since these were created, they're still popping up in derives (old redrows that get rerun, or new derives kicked off to create b/w pdfs).

Enable the luratech and grayscale modules to deal with these more gracefully: either convert to 8-bit depth before processing, or redrow with an informative error message, so the offending wm_jp2s can be removed and recreated.

(see also bug report on finding and eliminating the bad wm_jp2s)