Palette conversion out of sync with n64img

[marijnvdwerf]

Since decompals/n64img@fef013f, Splat performs rgba5551 (rgba16) to rgba8888 conversion using unpack_color, which was the same as the unpack_color that was removed in that commit. Now rgba-images have a different conversion method than indexed images.

To solve this, I propose to make n64img also export a new unpack_color function, and use that in both places.