Swift PNG is now MPL-2.0

Hi all, i’m happy to announce that with the agreement of the handful of other contributors to the project, the Swift PNG library is now available under the Mozilla Public License 2.0 (previously, it was GPL3). MPL2 is a weaker copyleft license that does not require all the code using the library to be released as open source, only the modifications to the library itself, if applicable. The patch release 3.0.1 carries the new license, as well as cleaning up many compiler warnings that have accumulated due to compiler drift during the ~1 1/2 years since the 3.0.0 release.