Support thumbnailing images in palette mode
Support thumbnailing images in palette mode

Fixes #1290

manthey/girder,Xarthisius/girder,essamjoubori/girder,jbeezley/girder,adsorensen/girder,RafaelPalomar/girder,kotfic/girder,salamb/girder,Xarthisius/girder,data-exp-lab/girder,jbeezley/girder,RafaelPalomar/girder,sutartmelson/girder,sutartmelson/girder,kotfic/girder,kotfic/girder,salamb/girder,salamb/girder,girder/girder,Kitware/girder,essamjoubori/girder,Xarthisius/girder,adsorensen/girder,RafaelPalomar/girder,Kitware/girder,girder/girder,adsorensen/girder,essamjoubori/girder,data-exp-lab/girder,kotfic/girder,Kitware/girder,Xarthisius/girder,data-exp-lab/girder,jbeezley/girder,sutartmelson/girder,manthey/girder,salamb/girder,Xarthisius/girder,salamb/girder,girder/girder,manthey/girder,essamjoubori/girder,Kitware/girder,data-exp-lab/girder,jbeezley/girder,adsorensen/girder,girder/girder,essamjoubori/girder,data-exp-lab/girder,sutartmelson/girder,kotfic/girder,sutartmelson/girder,manthey/girder,RafaelPalomar/girder,RafaelPalomar/girder,adsorensen/girder