Description: on an optimus system and neither graphic card backend appears to be functional with mpv playing mpeg* (2,3,4) encoded files; crash on not being able to find non-existent vdpau backend.

Additional info:
* package version(s): mpv 0.3.10-1
* config and/or log files:


Steps to reproduce:
[dcheung@hydra ~]$ ${optirun|primusrun|} mpv /var/local/media/hello.mp3
Playing: /var/local/media/hello.mp3
[ffmpeg/demuxer] mp3: Estimating duration from bitrate, this may be inaccurate
Detected file format: MP2/3 (MPEG audio layer 2/3) (libavformat)
...
[stream] Video (+) --vid=1 [P] '' (png)
[stream] Audio (+) --aid=1 (mp3)
Failed to open VDPAU backend libvdpau_i965.so: cannot open shared object file: No such file or directory
[vo/vdpau] Error when calling vdp_device_create_x11: 1
[vo/opengl] OpenGL context creation failed!
Segmentation fault (core dumped)