start_date must be set with the lock held. Otherwise, the condition
statement inside the ALSAThread while() startup loop may fail.
Once signaled, start_date belongs to the ALSA thread, so locking is not
needed anymore.

make explicit why using vlm crashes vlc
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

 It's not perfect, as they do not have the same dynamic, but it costs a lot
of CPU for something not that visible.
 If an option is wanted, it is easy to add.

It is not exact with rgb blending.

The name might be ill chosen, but duplicated code is worse.

recursive=expand can be too time-consuming (preventing users from exiting vlc)

We do not need to copy here anyway.

This kills a memory copy in most cases (FFMPEG input padding is the same
as VLC block padding, at least on my system).

is found (same function but not same data).
This can be usefull to find some errors.
This is only activated in debug mode.

 * Shrink the buffer in either direction before reallocating,
   so we don't need to copy discarded data.
(* Special case if the payload is entirely discarded.)
 * Avoid potentially overflowing pointer comparisons
  (In real life, it works, but it is not defined by the C specification).

And remove a lot of unnecessary code.