This does what it's documented to do.
The implementation reuses the code in mpv_detach_destroy(). Due to the
way async requests currently work, just sending a synchronous dummy
request (like a "ignore" command) would be enough to ensure
synchronization, but this code will continue to work even if this
changes.
The line "ctx->event_mask = 0;" is removed, but it shouldn't be needed.
(If a client is somehow very slow to terminate, this could silence an
annoying queue overflow message, but all in all it does nothing.)
Calling mpv_wait_async_requests() and mpv_wait_event() concurrently is
in theory allowed, so change pthread_cond_signal() to
pthread_cond_broadcast() to avoid missed wakeups.
As requested in issue #1542.