ENH: Changed 'char' to 'unsigned char' where used to represent the value of a pixel.  This should make the code portable to platforms on which 'char' is interpreted as 'signed char'.
ENH: Changed 'char' to 'unsigned char' where used to represent the value
of a pixel.  This should make the code portable to platforms on which
'char' is interpreted as 'signed char'.

thewtex/ImageViewer,KitwareMedical/ImageViewer,aylward/ImageViewer