update mpv and add support for vaapi

`python3 ${waf} configure --help` should auto-detect as stated in readme of mpv, but it just thinks that most of dependencies are detected, build then just silently ignores the so called false-autodetected dependencies, therefore I added a `optionalString` with `--enable-vaapi`.

Vaapi support can be enabled by `nixpkgs.config.mpv.vaapiSupport = true;`
Tested with Intel HD Graphics 4000 on i5, the cpu usage was halved, from 10% to 5%,
I think this is first vaapi supported video player in NixOS, I tried to make it work with mplayer (vaapi support not found), mplayer-vaapi (too much work, and last commit is half a year back) and vlc and was a no-go, I compiled it .. but watching videos made me want to take a dump - lagging and color deformation.

oh yea and do not forget to use: `$ mpv --hwdec=vaapi --vo=vaapi ./some/video.mp4`
and you have to have intel graphics (or possibly amd, not tested)