lavfi/vf_libplacebo: only drain actually used PTS

When combining multiple inputs, the output PTS may be less than the PTS
of the input. In this case, the current's code assumption of always
draining one value from the FIFO is incorrect. Replace by a smarter
function which drains only those PTS values that were actually consumed.

libavfilter/vf_libplacebo.c

@@ -971,6 +971,13 @@ static int handle_input(AVFilterContext *ctx, LibplaceboInput *input)
     return 0;
 }
 
+static void drain_input_pts(LibplaceboInput *in, int64_t until)
+{
+    int64_t pts;
+    while (av_fifo_peek(in->out_pts, &pts, 1, 0) >= 0 && pts <= until)
+        av_fifo_drain2(in->out_pts, 1);
+}
+
 static int libplacebo_activate(AVFilterContext *ctx)
 {
     int ret, retry = 0;
@@ -1023,8 +1030,7 @@ static int libplacebo_activate(AVFilterContext *ctx)
             ff_inlink_request_frame(in->link);
             return 0;
         case PL_QUEUE_OK:
-            if (!s->fps.num)
-                av_fifo_drain2(in->out_pts, 1);
+            drain_input_pts(in, out_pts);
             return output_frame(ctx, out_pts);
         case PL_QUEUE_ERR:
             return AVERROR_EXTERNAL;