It fixes a growing amount of cache as noticed by Sbastien Escudier.

It will allows to remove libvlc hack.

It is also needed for a clean API for libvlc.

It is thread safe (or should be).
No snapshot should be lost.
The image encoding is now done outside vout thread (in the callback).

It fixes a few memleaks in case of error.

(Feel free to move/cleanup)

In streams that do not have SPU ES setting spu-es to zero using libvlc_video_set_spu() results in playback being stopped. This is unwanted behaviour against which this patch protects now.

Fixes play/pause and fullscreen toggle.