image/png: do not apply filters if level is NoCompression

PNG filters are applied to get better compression ratio.
It does not make sense to apply them if we are not going
to compress.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://codereview.appspot.com/137830043

Committer: Nigel Tao <nigeltao@golang.org>