macosx: re-write and simplify the apple event handlers for GetURL and OpenURL, make it respect 'macosx-autoplay' (close #6319)

There may be more I forgot about..

This is necessary to compile with libav.git HEAD or with contribs.
Streaming output still needs fixing.

Note that the input coded buffer is now only freed if the decoding
returns a decoded frame, or the buffer has been entirely consumed
(see *pp_block = NULL).

If the user explicitly asks for a filter, it's an error to not be able
to instatiate it.
This reverts commit 7b0761c4.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

This is not supported in libavcodec anymore.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

We want to use encoder->fmt_in instead of original video size

Fixes: #8206

Now, S/PDIF works at least in the sense that it plays something which "seems"
to be in sync more or less.

refs #8083