It should fix a potential segfault when the vout is closed.

It allows removing partially a ugly input dependency in vout as well as
finer control at decoder side.

Now SSA subtitle pause is working even with karaoke.

This provides a pause with a maximal latency of one call
to pf_demux from the demuxer.

 I have modified spu_RenderSubpictures to take a b_paused argument.
 The calls to pf_pre_render/pf_update_regions could have been moved to
spu_SortSubpicture and might have been cleaner BUT the way used keeps
all time consumming tasks inside spu_RenderSubpictures.

As discussed on IRC. Yield was quite confusing.

It will avoid useless picture copy.

Do not move an already rendered subtitle.