Any Android specific modifications to upstream libpng (1.6.25) should
be listed here:

(1) Android has added the following files.
    pngusr.h
    Android.mk
    CleanSpec.mk
    MODULE_LICENSE_BSD_LIKE
    README.version

(2) pnglibconf.h
    This is copied from scripts/pnglibconf.h.prebuilt.
    Android has undefined PNG_WARNINGS_SUPPORTED.

(3) Removed contrib/testpngs/
    There is no reason to check in all of these files if we won't
    be using them.

(4) Applied the Intel SSE patch
    Upstream places the patch for SSE support
    in contrib/intel/intel_sse.patch.
    The patch was applied using the following command:
    'patch -i contrib/intel/intel_sse.patch -p1'

(5) Revert ADLER32 error handling to fix the infinite loop
Cherry picked from upstream:
https://github.com/glennrp/libpng/commit/d65a92b951079d315e17e20ba9e7b8423d19397e