Update libpng to 1.6.22rc01

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 801584656762224c5d7a36c39fb4470846607888..9324b4a15adfd617e7df8ece29f6af7534c8285a 100644
--- a/DEPS
+++ b/DEPS
@@ -18,7 +18,7 @@ deps = {
   "third_party/externals/nanomsg" : "https://skia.googlesource.com/third_party/nanomsg.git@0.4-beta",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@c4e33043fb071b6ea0a153845da625d7ed633d3d",
   # NOTE: If we update libpng, we may need to update the generated file at third_party/libpng/pnglibconf.h
-  "third_party/externals/libpng"  : "https://skia.googlesource.com/third_party/libpng.git@5765a2224946b953ec6702bd01205af475f8c80a",
+  "third_party/externals/libpng"  : "https://github.com/mattsarett/libpng.git@393063051764f2410daf931e8f414f37e157f7a1",

[msarett, 2016/05/16 14:14:16]

It's strange to clone from github/mattsarett.

We can't use the libpng mirror because there is now one extra config step.  We
need to run the following command to enable the Intel optimizations.

patch -i contrib/intel/intel_sse.patch -p1

[scroggo, 2016/05/16 19:20:45]

So does this mean we'll have to continue to use the github/mattsarett version?
I'm surprised that libpng is making us do a patch to enable these. It seems like
it would be cleaner to use a build flag? What does the patch look like*? Can we
instead add something to third_party/libpng/libpngconf.h [1]?

[1] https://skia.googlesource.com/skia/+/master/third_party/libpng/pnglibconf.h

I just answered my own question. It can be found here: [2]. It does the
following:
- Update configure.ac andMakefile.am
  - We don't use either of these, so do we need these changes?
- Modify pngpriv.h
  - Can we instead modify our pnglibconf.h? It looks like pngpriv.h includes it?

[2]
https://skia.googlesource.com/third_party/libpng/+/v1.6.22rc01/contrib/intel/...

[scroggo, 2016/05/16 21:16:09]

Matt said:
:(

My preference would be to update libpng to not require a patch, but I guess we
cannot have that for this version no matter what.

Please document in this file why we're pulling from your repository and what we
need to do to stop (and/or file a bug).

We should also have a README.google describing what our modifications are (and
why). Maybe that belongs in your repository, but perhaps we should also modify
the README we have checked into  third_party/libpng [1].

[1] https://skia.googlesource.com/skia/+/master/third_party/libpng/README.google

   "third_party/externals/giflib"  : "https://android.googlesource.com/platform/external/giflib.git@ab10e256df4f684260ca239905b1cec727181f6c",
 
   "third_party/externals/dng_sdk" : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",