image/png: always set up palette during DecodeConfig

The old code would decode the palette only for 8-bit
images during a DecodeConfig.
This CL keeps the behavior for 8-bit images and sets
up the decoded palette also for 1, 2 and 4-bit images.

Fixes issue 4279.

R=golang-dev, nigeltao
CC=golang-dev
https://codereview.appspot.com/7421048

Committer: Nigel Tao <nigeltao@golang.org>