SkPngFilters: guard with libpng >= 1.5.7

The read_filter array was added in libpng-1.5.7.
No point to these filter methods if we can't plug them into libpng.

TBR=msarett@google.com
NOTRY=true
NOPRESUBMIT=true

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1640933004

Review URL: https://codereview.chromium.org/1640933004
1 file changed