The convolver may be re-activated shortly after initialization
(e.g. session load, switching snapshots, or buffer-size.
In this case not all process threads may have started.
Convproc::stop_process() skips them (their state is still ST_IDLE).
Yet some short time later the thread's main function runs
and changes the state to ST_PROC, and check_stop () waits forever.
This is solved by waiting for all threads to start.
Ardour statically links libardour against zita-convolver,
this is required because it's used in plugins and available
by Lua scripts.
libzita-convolver has been modified to be in a custom namespace
and extern global symbols have been removed.
On most GNU/Linux distros system-wide default zita-convolver
is not compatible nor suitable to be used as-is.
