dispatch: wakeup only if needed on mp_dispatch_resume()

The wakeup is needed to make mp_dispatch_queue_process() return if
suspension is not needed anymore - which is only the case when the
request count reaches 0.

The assertion added with this commit always has/had to be true.
command: allow native access to "vf" property

This allows client API users and Lua scripts to side-step the pretty
horrible video filter string "language" (although it's back and can't be
avoided when using libavfilter).
lua: fix stack going out of sync

This broke with recursive tables.