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.

A better solution than ObjectKillChildrens is really needed.

The playlist has to be locked before calling it.
Another way could be to change playlist_IsEmpty prototype to have a b_locked
parameter (like others playlist functions).

It fixes title display with vout recycling.
It removes one vlc_object_find.

It should fix a few issues with embed video and fullscreen controler (the
widget may? not yet be ready in time).
It fixes dialog provider.

It now works in non embed mode.
It follows multiple vouts (not perfect yet).

It will be used to properly attach/detach the fullscreen controller
independantly of embed vout.