lua: give more control over timers

Now they can be paused and resumed.

Since pausing and disabling the timer is essentially the same underlying
operation, we also just provide one method for it.

mp.cancel_timer probably still works, but I'm considering this
deprecated, and it's removed from the manpage. (We didn't have a release
with this function yet, so no formal deprecation.)
remove redundant/outdated line from encodeing.rst:"what works"
vo_vdpau: more debugging output

Might help to debug certain problems with Mesa.
Merge branch 'master' of https://github.com/mpv-player/mpv