improve artifacting by expanding the color palette

[willnorris]

currently, transformed images use the same color palette as the original image. This may cause some artifacting on resized images because a pixel may use a color average of its surrounding pixels that is not in the palette.

This is the same as gifsicle's "resize-colors" option:

Allow Gifsicle to add intermediate colors when resizing images. Normally, Gifsicle's resize algorithms use input images' color palettes without changes. When shrinking images with very few colors (e.g., pure black-and-white images), adding intermediate colors can improve the results.