If there is a parent input, it cannot go away. There was no point in
reference counting.

This is arguably the only case where the callback made sense.

(The read callback is as suspicious and retained.)

In most cases, there is no or little room for optimizing this, and
the benefits are minimal. This provides a generic implementation so
that each stream filter does not need to reinvent the wheel.

Depending on device and audio format, AudioTrack can't hold an audio buffer big
enough.

This commit adds a circular buffer and a thread that will play the data to
AudioTrack coming from this circular buffer. This commit also decrease the
AudioTrack internal buffer size.

We can use the same buffer since the buffer position is moved by the AudioTrack
write call.

Wait for a fixed time since the sleep duration is not constant anymore.

Usage:

$ vlc concat:// :concat=URL1,URL2,....URLn

Note: As for the existing input-slave and input-list options, URLs are
assumed to not contain unencoded commas. This is usually the case but
not strictly warranted as the comma is a sub-delimiter.