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>
diff --git a/DEPS b/DEPS
index 143dd11..f6ce195 100644
--- a/DEPS
+++ b/DEPS
@@ -12,7 +12,7 @@
   "third_party/externals/imgui"         : "https://github.com/ocornut/imgui.git@6384eee34f08cb7eab8d835043e1738e4adcdf75",
   "third_party/externals/jsoncpp"       : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
   "third_party/externals/libjpeg-turbo" : "https://skia.googlesource.com/third_party/libjpeg-turbo.git@6de58e0d28014caf2fc1370145f22fd6d65f63e3",
-  "third_party/externals/libpng"        : "https://skia.googlesource.com/third_party/libpng.git@v1.6.32",
+  "third_party/externals/libpng"        : "https://skia.googlesource.com/third_party/libpng.git@v1.6.33",
   "third_party/externals/libwebp"       : "https://chromium.googlesource.com/webm/libwebp.git@v0.6.0",
   "third_party/externals/microhttpd"    : "https://android.googlesource.com/platform/external/libmicrohttpd@748945ec6f1c67b7efc934ab0808e1d32f2fb98d",
   "third_party/externals/piex"          : "https://android.googlesource.com/platform/external/piex.git@8f540f64b6c170a16fb7e6e52d61819705c1522a",