Make png filter functions compatible with libpng

We hope to upstream these functions to libpng (a C library).

Let's make the code that Skia tests on look like the code
that we are submitting.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1699953002

Review URL: https://codereview.chromium.org/1699953002
1 file changed