PalettedColorModel was renamed to color.Palette
over three years ago by https://golang.org/cl/5132048.

Change-Id: I0204ade10eabff45620fda2990fed428c65d871e
Reviewed-on: https://go-review.googlesource.com/3305Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>