filter: decide how multi-threading is supposed to work

Instead of vague ideas about making different filter graphs on different
threads interact directly, this have no direct support. Instead, helpers
are required (such as added with the next commit).

Document it. Different root filters (i.e. separate filter graphs) are
now considered to be part of separate threads, so assert() if they're
found to accidentally interact.

filters/filter.c

@@ -183,6 +183,8 @@ bool mp_filter_run(struct mp_filter *filter)
 
     r->filtering = true;
 
+    // Note: some filters may call mp_filter_wakeup() from process on themselves
+    //       to queue a wakeup again later. So do not call this in the loop.
     flush_async_notifications(r);
 
     while (r->num_pending) {
@@ -311,6 +313,8 @@ static struct mp_pin *find_connected_end(struct mp_pin *p)
 // state flags.
 static void init_connection(struct mp_pin *p)
 {
+    struct filter_runner *runner = p->owner->in->runner;
+
     if (p->dir == MP_PIN_IN)
         p = p->other;
 
@@ -321,6 +325,12 @@ static void init_connection(struct mp_pin *p)
     assert(!in->user_conn);
     assert(!out->user_conn);
 
+    // This and similar checks enforce the same root filter requirement.
+    if (in->manual_connection)
+        assert(in->manual_connection->in->runner == runner);
+    if (out->manual_connection)
+        assert(out->manual_connection->in->runner == runner);
+
     // Logicaly, the ends are always manual connections. A pin chain without
     // manual connections at the ends is still disconnected (or if this
     // attempted to extend an existing connection, becomes dangling and gets
@@ -339,6 +349,7 @@ static void init_connection(struct mp_pin *p)
         assert(!cur->data.type); // unused for in pins
         assert(!cur->other->data_requested); // unset for unconnected out pins
         assert(!cur->other->data.type); // unset for unconnected out pins
+        assert(cur->owner->in->runner == runner);
         cur->within_conn = cur->other->within_conn = true;
         cur = cur->other->user_conn;
     }