lavfi/vf_libplacebo: generalize frame update to multiple inputs

In the event that some frame mixes are OK while others are not, the
priority goes:

1. Errors in updating any frame -> return error
2. Any input incomplete -> request frames and return
3. Any inputs OK -> ignore EOF streams and render remaining inputs
4. No inputs OK -> set output to most recent status

This logic ensures that we can continue rendering the remaining streams,
no matter which streams reach their end of life, until we have no
streams left at which point we forward the last EOF.

libavfilter/vf_libplacebo.c

@@ -980,9 +980,8 @@ static void drain_input_pts(LibplaceboInput *in, int64_t until)
 
 static int libplacebo_activate(AVFilterContext *ctx)
 {
-    int ret, retry = 0;
+    int ret, ok = 0, retry = 0;
     LibplaceboContext *s = ctx->priv;
-    LibplaceboInput *in = &s->inputs[0];
     AVFilterLink *outlink = ctx->outputs[0];
     int64_t pts, out_pts;
 
@@ -1014,9 +1013,12 @@ static int libplacebo_activate(AVFilterContext *ctx)
                 return 0;
         }
 
-        if (s->status && out_pts >= s->status_pts) {
-            ff_outlink_set_status(outlink, s->status, s->status_pts);
-            return 0;
+        /* Update all input queues to the chosen out_pts */
+        for (int i = 0; i < s->nb_inputs; i++) {
+            LibplaceboInput *in = &s->inputs[i];
+            if (in->status && out_pts >= in->status_pts) {
+                in->qstatus = PL_QUEUE_EOF;
+                continue;
             }
 
             in->qstatus = pl_queue_update(in->queue, &in->mix, pl_queue_params(
@@ -1028,13 +1030,27 @@ static int libplacebo_activate(AVFilterContext *ctx)
             switch (in->qstatus) {
             case PL_QUEUE_MORE:
                 ff_inlink_request_frame(in->link);
-            return 0;
+                retry = true;
+                break;
             case PL_QUEUE_OK:
-            drain_input_pts(in, out_pts);
-            return output_frame(ctx, out_pts);
+                ok = true;
+                break;
             case PL_QUEUE_ERR:
                 return AVERROR_EXTERNAL;
             }
+        }
+
+        if (retry) {
+            return 0;
+        } else if (ok) {
+            /* Got any valid frame mixes, drain PTS queue and render output */
+            for (int i = 0; i < s->nb_inputs; i++)
+                drain_input_pts(&s->inputs[i], out_pts);
+            return output_frame(ctx, out_pts);
+        } else if (s->status) {
+            ff_outlink_set_status(outlink, s->status, s->status_pts);
+            return 0;
+        }
 
         return AVERROR_BUG;
     }