Import mpv-0.4.2 as wip/mpv.

mpv is a movie player based on MPlayer and mplayer2. It supports
a wide variety of video file formats, audio and video codecs, and
subtitle types.

TODO:
* make waf a DISTFILE instead of files/waf
* use pkgsrc config file framework
* find out why playing videos on NetBSD is too slow and audio is distorted