filter: reduce redundant re-iterations

When the player core requests new frames from the filter, this is called
external/recursive filtering, which acts slightly differently from when
filters request new data internally. Mainly this is so the external user
doesn't have to call mp_filter_graph_run() just to get a frame. This
causes a number of complications, and the short version is that until
now, mp_filter_graph_run() has unnecessarily returned true in the
current common case, which made the playloop run too often for no
reason.

The problem is that when a mp_pin is read externally, updating the same
mp_pin during recursive filtering flagged external_pending when the
result was written, which made mp_filter_graph_run() return true, which
made the playloop call mp_filter_graph_run() again. This is redundant
because the caller is obviously checking the new state of the mp_pin
immediately.

The only situation in which external_pending really must be set is if
_another_ pin is changed. In theory, we could also unset it if the set
of "external" pins that are not in a signaled state becomes empty, but
we don't track that in a convenient way.

This commit removes the redundant signaling, and avoids running the
playloop an additional time for each video and audio frame (as it
actually was planned from the beginning, but duh).

filters/filter.c

@@ -74,6 +74,9 @@ struct filter_runner {
     // If we're currently running the filter graph (for avoiding recursion).
     bool filtering;
 
+    // If set, recursive filtering was initiated through this pin.
+    struct mp_pin *recursive;
+
     // Set of filters which need process() to be called. A filter is in this
     // array iff mp_filter_internal.pending==true.
     struct mp_filter **pending;
@@ -131,10 +134,21 @@ static void add_pending(struct mp_filter *f)
     // something naive and dumb does the job too.
     f->in->pending = true;
     MP_TARRAY_APPEND(r, r->pending, r->num_pending, f);
+}
+
+static void add_pending_pin(struct mp_pin *p)
+{
+    struct mp_filter *f = p->manual_connection;
+    assert(f);
+
+    if (f->in->pending)
+        return;
+
+    add_pending(f);
 
     // Need to tell user that something changed.
-    if (f == r->root_filter)
-        r->external_pending = true;
+    if (f == f->in->runner->root_filter && p != f->in->runner->recursive)
+        f->in->runner->external_pending = true;
 }
 
 // Possibly enter recursive filtering. This is done as convenience for
@@ -143,8 +157,9 @@ static void add_pending(struct mp_filter *f)
 // stacks.) If the API users uses an external manually connected pin, do
 // recursive filtering as a not strictly necessary feature which makes outside
 // I/O with filters easier.
-static void filter_recursive(struct mp_filter *f)
+static void filter_recursive(struct mp_pin *p)
 {
+    struct mp_filter *f = p->conn->manual_connection;
     assert(f);
     struct filter_runner *r = f->in->runner;
 
@@ -152,9 +167,15 @@ static void filter_recursive(struct mp_filter *f)
     if (r->filtering)
         return;
 
+    assert(!r->recursive);
+    r->recursive = p;
+
     // Also don't lose the pending state, which the user may or may not
     // care about.
     r->external_pending |= mp_filter_graph_run(r->root_filter);
+
+    assert(r->recursive == p);
+    r->recursive = NULL;
 }
 
 void mp_filter_internal_mark_progress(struct mp_filter *f)
@@ -263,8 +284,8 @@ bool mp_pin_in_write(struct mp_pin *p, struct mp_frame frame)
     assert(p->conn->data.type == MP_FRAME_NONE);
     p->conn->data = frame;
     p->conn->data_requested = false;
-    add_pending(p->conn->manual_connection);
-    filter_recursive(p->conn->manual_connection);
+    add_pending_pin(p->conn);
+    filter_recursive(p);
     return true;
 }
 
@@ -282,9 +303,9 @@ bool mp_pin_out_request_data(struct mp_pin *p)
     if (p->conn && p->conn->manual_connection) {
         if (!p->data_requested) {
             p->data_requested = true;
-            add_pending(p->conn->manual_connection);
+            add_pending_pin(p->conn);
         }
-        filter_recursive(p->conn->manual_connection);
+        filter_recursive(p);
     }
     return mp_pin_out_has_data(p);
 }
@@ -292,7 +313,7 @@ bool mp_pin_out_request_data(struct mp_pin *p)
 void mp_pin_out_request_data_next(struct mp_pin *p)
 {
     if (mp_pin_out_request_data(p))
-        add_pending(p->conn->manual_connection);
+        add_pending_pin(p->conn);
 }
 
 struct mp_frame mp_pin_out_read(struct mp_pin *p)