Edit colors on images with adaptive palettes.

Edit colors on images with adaptive palettes.

In Zork Zero and Arthur, some images have no palettes of their own.
These change their colors according to images plotted before.  Kevin
Bracy called these "adaptive palette pictures".  To deal with them, he
proposed an extension to the Blorb standard to add an "APal" chunk which
lists the images that have adaptive palettes.  The changes in this
commit make sfrotz read that chunk and then rewrite the palettes of
listed images when they are loaded.

This is described in much greater detail in
http://ifarchive.org/if-archive/programming/blorb/blorb-infocom-extension.txt

The work to correctly implement processing of adaptive palettes was done by Paul
Gilbert <dreammaster@scummvm.org>.  I, David Griffith, condensed what would have been
an irritatingly messy merge into a single commit and did general cleanup before
committing Paul's changes using his name.