Blackthumbnails dev 4 4

Same as gh-1871

log in as user-6 group read-only -1 (omero4-demo.openmicroscopy.org port 14064)
  * add user-9 to the tree
  * open one of the gatan image owned by user-9
  * the thumbnail should no longer be black 

see https://trac.openmicroscopy.org.uk/ome/ticket/11776