liblbx: Don't scale palette values internally.

Output the 18-bit palette values unadulterated, leaving it up to the
users to do conversion if necessary.  Push the conversion down into the
consumers (currently PNG output and the GTK GUI) where a 24-bit palette
is required.

While we're at it, adjust the conversion to scale to the full output
range of [0, 255].  This is a slight change to the output, so all the
image hashes need to be adjusted in the regression test.