This saves memory, diminishes latency and fixes an unlikely loss of
events if too many actions are sent. This may expose some re-entrancy
bugs, although I did some succesful smoke tests. Any path that emits a
key action or a key press needs to be careful. vout_thread_t.pf_manage
is the most obvious case. That should be safe as the vout thread holds
no locks when in pf_manage. Not sure about OpenGL though.

A small bug with SPU across vout change remains present as before.