manpage: improve --osd-level description
stream: remove unused functions

Interestingly, their last use was removed with commit bbbea793, over
4 months ago.

Also remove a stray struct demux_stream forward declaration.
stream: use uninterruptible sleep on reconnecting

This is the only function which actually used the time argument of
stream_check_interrupt(). Considering that the whole player freezes
anyway, this is not worth the complication.

Also generally reduce the maximum wait time due to timeout. Introduce
exponential backoff, which makes the first reconnect retries faster, but
still waits up to 500ms in the later retries.
stream: remove interrupt callback global variables

This used global variables for the asynchronous interrupt callback.

Pick the simple and dumb solution and stuff the callback into
mpv_global. Do this because interrupt checking should also work in the
connect phase, and currently stream creation equates connecting.
Ideally, this would be passed to the stream on creation instead, or
connecting would be separated from creation. But since I don't know yet
which is better, and since moving stream/demuxer into their own thread
is something that will happen later, go with the mpv_global solution.
stream: make mp_input_check_interrupt thread-safe

The interrupt callback will can be called from another thread if the
cache is enabled, and the stream disconnects. Then stream_reconnect()
will call this function from within the cache thread.

mp_input_check_interrupt() is not thread-safe due to read_events() not
being thread-safe. It will call input callbacks added with
mp_input_add_fd() - these callbacks lead to code not protected by locks,
such as reading X11 events.

Solve this by adding a stupid hack, which checks whether the calling
thread is the main playback thread (i.e. calling the input callbacks
will be safe). We can remove this hack later, but it requires at least
moving the VO to its own thread first.
stream: use libavformat interrupt callback

This will allow to cancel opening slow/stuck network streams faster. How
well his work probably depends on the protocol implementation in
libavformat.
vf_pp: PP_PICT_TYPE_QP2 is always defined