It's unclear how exactly a
colormapped image gets to this
spot given the tendency of other
image processing tools to flatten
such images, but someone made it happen, so now we make sure
the image is okay.
Closes#262
Turns out it does not transcode at all in this case, so probably going
to revert to transcoding PNG -> PNG. However if pngquant or similar is
done, this API will still be useful.
Leptonica does a better job of encoding them than Ghostscript, about -15%.
For a test file 450k worth of
PNGs was reduced to 388k with no loss of quality.
James R. Barlow