Fix channel count when exporting RGB images in ImageJ (rebased onto dev_5_0)



This is the same as gh-1175 but rebased onto dev_5_0.

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Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12091 and http://trac.openmicroscopy.org.uk/ome/ticket/11569.

To test, verify that both test cases outlined in the tickets are now working.  It would also be helpful to verify that exporting a non-RGB image with multiple channels still works as expected.