mp4v2: Switch to maintained fork, update to 2.1.3

[ryandesign (Ryan Carsten Schmidt)]

We are using an unmaintained fork of mp4v2 with ​known problems. In that issue, it is pointed out that we might switch to ​this new maintained fork as other distributions have done. This is a good idea. Since this switch will decrease the version from 4.1.3 to 2.1.3, the epoch must once again be increased. Before committing any such switch, I recommend to rebuild and test any port that uses mp4v2 to verify that it still finds mp4v2 and works. Read the commit message from when we downgraded to version 4.1.3 for some background.