Use Zlib 1.2.7 (patch from Niall Moody)

CMake/ExternalPNG.cmake

@@ -16,8 +16,8 @@ EXTERNALPROJECT_ADD(zlib
     PREFIX ${libpng_PREFIX}
 
     DOWNLOAD_DIR ${POLYCODE_DEPS_DOWNLOAD_DIR}
-    URL http://zlib.net/zlib-1.2.6.tar.gz
-    URL_MD5 618e944d7c7cd6521551e30b32322f4a
+    URL http://zlib.net/zlib-1.2.7.tar.gz
+    URL_MD5 60df6a37c56e7c1366cca812414f7b85
 
     PATCH_COMMAND ${CMAKE_COMMAND} -E remove <SOURCE_DIR>/zconf.h