Vout rework - lighten render time - Move snapshot taking code

Snapshot taking code IMO should be moved from the vout thead. Whenever a snapshot is taken late picture skipped messages spit out. I suggest to move snapshot taking code (e.g. the caller of vout_Snapshot) to input thread ( or libvlc user thread) by taking the snapshot in the snapshotCallback.

This will lighten a little vout render loop that is very sensitive to latency.

I can implement this if the logic is ok.