Partial conversion usage of BUT files to PNG files

Remove reported unused local variables

This commit replaces all of the straightforward image file usage.
Images that have specific sub-frames loaded still need to be replaced.
Redefine palette copy operation

You now specify a range of color indices as (start, end). More
importantly, the copy operation no longer has the ability to change
color indices: color N on the source will always correspond to
color N on the destination.
Ensure that but2png is linked with zlib