libpng: update to 1.6.32, pull using DEPS

I'd have done this as two steps, but we didn't have a pure copy of
libpng to start with.  The patches we did have, though, have been
upstreamed and are now unneeded.

Change-Id: I884b9bc47afe5000f5a521f66a3bb95c0411b39a
Reviewed-on: https://skia-review.googlesource.com/48620
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
39 files changed