fix incorrect colors in Lefty postscript output

fix incorrect colors in Lefty postscript output

This code intended to print color data to its output file in postscript (PS)
format. However it miscalculated the low nibble of each non-zero byte as 1 due
to an incorrect operator. This may have not been noticed in the resulting image
due to low precision. This was exposed by the compiler flag -Wlogical-op. Fixes
#1927.

At first glance, it looks like this could be written without shifting and
masking at all by simply printing the byte padded to width 2, but maybe I am
missing some subtlety.