DICOM: fix byte skipping for >8 bit data

As noted in the comments, this adjusts the byte skipping as needed to
ensure that each plane has the correct number of decompressed bytes.

Fixes http://trac.openmicroscopy.org/ome/ticket/12624.  To test, verify that the ```RLE.dicom``` file attached to http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-November/004863.html opens the same in Bio-Formats as it does in Osirix.  There should be no build failures as a result of this change.