Signed-off-by: NMiguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: NCosmin Truta <ctruta@gmail.com>

The new libpng license comprises the terms and conditions from the zlib
license, and the disclaimer from the Boost license.

The legacy libpng license, used until libpng-1.6.35, is appended to the
new license, following the precedent established in the Python Software
Foundation License version 2.

From now on, the list of contributing authors shall be maintained in a
separate AUTHORS file.

The lists of previous contributing authors, mentioned in the legacy
libpng license and considered to be an integral part of that license,
are kept intact, with no further updates.

Also reformat the license text.