MRC: fix handling of mode 16 data

Mode 16 denotes 8-bit RGB data, not 16-bit data.  See
https://trello.com/c/guQdSW8V/235-follow-ups-with-joaquin
Merge pull request #1091 from melissalinkert/mrc-pixel-type

MRC: fix handling of mode 16 data
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Repository: openmicroscopy/bioformats
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