histogram is equalized or unmodified.  This way, if you are looking at
particularly dark or badly contrasted images you can toggle this mode and maybe
get a better view of what you are labeling.

--HG--
rename : tools/imglab/src/image_dataset_metadata.cpp => dlib/data_io/image_dataset_metadata.cpp
rename : tools/imglab/src/image_dataset_metadata.h => dlib/data_io/image_dataset_metadata.h

first image.