vf_vapoursynth: handle destruction more gracefully

We were relying on vsscript_freeScript() to take care of proper
termination. But it doesn't do that: it doesn't wait for the filters to
finish and exit at all. Instead, it just destroys all objects, which
causes the worker threads to crash sometimes.

Also, we're supposed to wait for the frame callback to finish before
freeing the associated node.

Handle this by explicitly waiting as far as we can. Probably fixes
crashes on seeking, although VapourSynth itself might also need some
work to make this case completely stable.

video/filter/vf_vapoursynth.c

@@ -307,18 +307,20 @@ static void destroy_vs(struct vf_instance *vf)
 {
     struct vf_priv_s *p = vf->priv;
 
+    // Wait until our frame callback returns.
+    pthread_mutex_lock(&p->lock);
+    p->shutdown = true;
+    pthread_cond_broadcast(&p->wakeup);
+    while (p->getting_frame)
+        pthread_cond_wait(&p->wakeup, &p->lock);
+    pthread_mutex_unlock(&p->lock);
+
     if (p->in_node)
         p->vsapi->freeNode(p->in_node);
     if (p->out_node)
         p->vsapi->freeNode(p->out_node);
     p->in_node = p->out_node = NULL;
 
-    pthread_mutex_lock(&p->lock);
-    p->shutdown = true;
-    pthread_cond_broadcast(&p->wakeup);
-    pthread_mutex_unlock(&p->lock);
-
-    // Expect that this properly waits until all filters return etc.
     if (p->se)
         vsscript_freeScript(p->se);
 
@@ -326,7 +328,6 @@ static void destroy_vs(struct vf_instance *vf)
     p->vsapi = NULL;
     p->vscore = NULL;
 
-    assert(!p->getting_frame);
     assert(!p->in_node_active);
 
     p->shutdown = false;