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 #1108 from melissalinkert/rebased/develop/mrc-pixel-type

MRC: fix handling of mode 16 data (rebased onto develop)