MFH: r501313

multimedia/dav1d: update to 0.3.1

Changes: https://code.videolan.org/videolan/dav1d/tags/0.3.1
Changes: https://code.videolan.org/videolan/dav1d/blob/0.3.1/NEWS
Changes: https://code.videolan.org/videolan/dav1d/compare/0.3.0...0.3.1
Approved by: ports-secteam (required by Firefox 68)