It fixes embed video at start.

Pointed out by Courmish.

It eases security review.

It fixes item name with xpsf playlist (at least).

Otherwise the receiver would -in principle- try to receive RTP on the
next (plus one) even port, whereas the m= line port would be used for
RTCP. We could also pass a=rtcp for the normal even/RTP & odd/RTCP
ports setup, but that can only confuse receivers with no gain.

Note that receivers may not parse the a=rtcp parameter in any case. In
fact, VLC does not parse the a=rtcp field to this day. In any case,
sending RTP to an *even* port is the only safely interoperable option.

Pointed out by Dnumgis.

Without it, input_ressource_HoldVout and vout windows request could deadlock.

This is consistent with the comments in <vlc_block.h>.
So far, cancellation would only occur when sleeping was needed.

As pointed out by Laurent, cancellation was not safe here

Thanks j-b for the infos.