- Update waf to 1.9.1

- Force rebuilding with every minor update of ffmpeg

- Update to version 0.18.1
* Note: Running mpv with different versions of the
FFmpeg/Libav libraries than it was compiled with
is no longer supported. Even supposedly ABI-compatible
versions have been a source of trouble, and it creates
far too much complexity with little to no benefit,
coupled with absurd and unusable FFmpeg API artifacts.
Instead, mpv will exit with an error when such a
situation is detected.
This simply means that mpv needs to be rebuilt whenever
FFmpeg libraries change.
New Features:
* d3d: implement screenshots for --hwdec=d3d11va
* vo_opengl: add output_size uniform to custom shader
* vo_opengl: implement the Panasonic V-Log function
(#3157)
* vo_opengl: implement ARIB STD-B68 (HLG) HDR TRC
Options and oommands:
* command: pack sub image data in overlay-add command
* options: deprecate --heartbeat-cmd
* audio: deprecate --softvol
* audio: drop --softvol=no and --softvol=auto (#3322)
Fixes and minor enhancements:
* video: fix deinterlace filter handling on pixel
format changes
* x11: silence xdg-screensaver