Any Android specific modifications to upstream libpng (1.6.20) 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) PNG_INDEX_SUPPORTED optimizations.