libpng: Upgrade to 1.6.25

The changes from 1.6.22 (beta) to
1.6.25 (final) include many general fixes,
fixes for memory leak issues and optimizations
in order to improve performance.
Also upstream added support for the MIPS
platform.

Support for Intel SSE isn't built in any more.
For support a patch in contrib/intel/intel_sse.patch
has to be applied. This patch additionally
documents this in README.android as an Android
specific addition.

The patch for Intel SSE support
from upstream applied cleanly.

Libpng 1.6.25 released on September 1, 2016.

Test: built for ARM and checked whether NEON optimizations were compiled
      built for x86/x86_64 and checked whether SSE optimizations were compiled

Change-Id: If647f42a1fb4d7022286010d262d287dac4030df
Signed-off-by: Alex Naidis <alex.naidis@linux.com>
diff --git a/README b/README
index 4c5b0ec..17bca9a 100644
--- a/README
+++ b/README
@@ -1,4 +1,4 @@
-README for libpng version 1.6.22beta03 - February 8, 2016 (shared library 16.0)
+README for libpng version 1.6.25 - September 1, 2016 (shared library 16.0)
 See the note about version numbers near the top of png.h
 
 See INSTALL for instructions on how to install libpng.
@@ -180,15 +180,18 @@
       pngwutil.c    =>  Write utility functions
       arm           =>  Contains optimized code for the ARM platform
       contrib       =>  Contributions
+       arm-neon         =>  Optimized code for ARM-NEON platform
        examples         =>  Example programs
        gregbook         =>  source code for PNG reading and writing, from
                             Greg Roelofs' "PNG: The Definitive Guide",
                             O'Reilly, 1999
+       intel            =>  Optimized code for INTEL-SSE2 platform
        libtests         =>  Test programs
        pngminim         =>  Minimal decoder, encoder, and progressive decoder
                             programs demonstrating use of pngusr.dfa
        pngminus         =>  Simple pnm2png and png2pnm programs
        pngsuite         =>  Test images
+       testpngs
        tools            =>  Various tools
        visupng          =>  Contains a MSVC workspace for VisualPng
       projects      =>  Contains project files and workspaces for