audio filter plugins are now useless.
This breaks visual (needs to be ported to filter_t) and will likely
expose bugs in under-used audio filter2 infrastructure.

TODO: use filter chain instead of custom filter handling in core.

Big filter changes, and worse, moved audio_format plugin.

Signed-off-by: Rafaël Carré <rafael.carre@gmail.com>

This migrates to a filter_t-like syntax

Open fails if p_sys is NULL.

For lack of vout display currently doing it (obviously ;), it is
untested.

It is needed to avoid problems with threaded event.

(We have a warning for unknown event types anyway)

There is no functionnal changes but it will be needed to reduce locking time
when vout display are allowed to emit events from their own thread.

This slows down the video output thread too much, and does not belong in
the video output plugin anyway. It should go to the screensaver.
(I have also have a patch to move it to xcb_window if someone wants it).