Update multimedia/mpv to 0.15.0.

Changes:
Release 0.15.0
==============
Build System Changes
--------------------
- OS X bundle: remove git sha from the Info.plist version (issue #2677)
- add "lua51" ("51obsd") to list of possible lua names
- add option to customize config files system path (issue #2704)

Features
--------
New
~~~
- vo_opengl: implement support for transparent video display on OS X (alpha=yes suboption)
- vo_opengl: use a checkerboard pattern as background for transparent video by default

Options and Commands
--------------------
Added
~~~~~
- add --audio-file-paths (issue #2632)
- player, stream_dvb: implement dvb-channel-name property, add switch binding

Changed
~~~~~~~
- vf_stereo3d: add alternating modes
- vo_opengl: disable pbo by default for opengl-hq due to driver problems
- vf_yadif: change defaults (issue #2539)
- command: change heuristic for files with 1 chapter (issue #2550)
- demux_mkv: adjust subtitle preroll defaults
- exclude 360 from --video-rotate range (issue #2647)
- osd: make osd-width/height properties watchable

Fixes and Minor Enhancements
----------------------------
- ao_pulse: check for sample rate bounds, attempt fallback (issue #2654)
- ao_wasapi: remove volume "restore" on exit
- demux_cue: better error resilience
- mixer: fix volume initialization with --af=volume
- mpv.desktop: add audio/mp4 mime type
- player: detect audio PTS jumps, make video PTS heuristic less aggressive
- player: make watch later/resume work when "playing" directories
- player: reset playback abort when reloading a file (issue #2568)
- recognize frame sequenced 3D Matroska video
- stream_dvb: fix channel switching
- vaapi: add VP9 profile (requires VA-API 0.38.1 or newer)
- vo_opengl: dxinterop: prevent crash after lost device
- vo_opengl: enable brightness/contrast controls for RGB
- vo_opengl: fix operation on GLES 2.0
- vo_opengl: fix operation on GLSL versions earlier than 1.30
- vo_opengl: flip screenshot image if backend uses flipped rendering (issue #2635)
- vo_opengl: reset nnedi3 weights properly (issue #2661)
- vo_rpi: handle rotation
- vo_rpi: work around firmware oddness leading to incorrect video rect
- windows: fix fd://
- ytdl: Include Referer header as well
- TOOLS/zsh.pl: add .opus extension in zsh completions


(leot)