image/png: improve compression by skipping filter for paletted images

image/png: improve compression by skipping filter for paletted images

Compression of paletted images is more efficient if they are not filtered.
This patch skips filtering for cbP8 images.
The improvements are demonstrated at https://github.com/olt/compressbench

Fixes #16196

Change-Id: Ie973aad287cacf9057e394bb01cf0e4448a77618
Reviewed-on: https://go-review.googlesource.com/29872
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>