vout: always pass vout_control_Hold()

A thread calling vout_control_Hold() may only pass while vout_control_Pop() is waiting.

Since f1bf7ce5, in practice, it almost never waits, so vout_control_Hold() may be blocked for a long time.

If there is at least one thread waiting on vout_control_Hold(), make the vout thread wait from vout_control_Pop().

Fixes #25694 (closed)