These are test images for gif2png.

ducks.gif and gtcd.gif are paletted color images.

ten-wire.gif is unusual in that it is grayscale, but comes out as a smaller
PNG if paletted.  1.1.0 screwed up this case because remapping wasn't turned
off along with the grayscale flag; this clobbered the color table.

The Makefile supports performing regression tests of the conversion.
