Pointed-out-by: Sébastien Escudier

Crash happens if you do:
 - Play item
 - Activate vfilter
 - Stop item
 - Deactivate vfilter... BooM.
(cherry picked from commit c876401791144309d2427214d818a34198238358)

(cherry picked from commit c31d99fdf67fbc9060fb9f413d06b0acf4efc1b6)

Already available in the interface sprefs.
This reverts commit c1a144df.

We were already blending directly on top of the destination
picture. This simplifies the code and should prevent further
confusion.

(it was used by net_OpenDgram when no server is specified).

This reverts commit 9a5e5236.
Not intended for master.

SleepEx expects a duration in milli second.

Make sure that we don't add the briged elementary streams to the next
stream output chain element when in placeholder mode. Maybe I should
split normal bridge-in and placeholder mode because the code is
begining to be filled with quite a few if( p_sys->b_placeholder ).

Add default enabled option to force placeholder state toggling to
happen only on iframes. This removes artifacts at the expense of a
longer switch delay.