roll libpng to 1.6.33

Apparently 1.6.34 has been released too, but I can't find it in Git, and
1.6.33 is the one with that patch we're really interested in, right?

Let's also just #include the prebuilt pnglibconf.h to make it clear that
it's unchanged from the original?

Change-Id: Ia415486f30c7aff1575f96add8edce855eef9207
Reviewed-on: https://skia-review.googlesource.com/54040
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed