Allow use of inputs without DEMUX_GET/SET_*TIME in input-slave.

Allow use of inputs without DEMUX_GET/SET_*TIME in input-slave.

This is commit is necessary to split the audio and video capture accesses.
Example use with current git master to record screen and webcam:
./vlc screen:// --screen-fps 25 --screen-width 100 --screen-height 100 -I dummy --screen-follow-mouse --input-slave v4l2://:audio-method=0 -vv --sout '#transcode{vcodec=mp2v,vb=1024}:std{access=file,mux=ts,dst=/tmp/a.ts}' --run-time 10