filter/ffmpeg: fill AVFrame::pts

Some libavfilter plugins don't produce any output if `pts` is never
set, e.g. the `lowpass` plugin.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/2114

src/filter/plugins/FfmpegFilter.cxx

@@ -66,6 +66,9 @@ FfmpegFilter::FilterPCM(std::span<const std::byte> src)
 #endif
 	frame->nb_samples = src.size() / in_audio_frame_size;
 
+	frame->pts = pts;
+	pts += frame->nb_samples;
+
 	frame.GetBuffer();
 
 	memcpy(frame.GetData(0), src.data(), src.size());

src/filter/plugins/FfmpegFilter.hxx

@@ -1,14 +1,15 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 // Copyright The Music Player Daemon Project
 
-#ifndef MPD_FFMPEG_FILTER__HXX
-#define MPD_FFMPEG_FILTER__HXX
+#pragma once
 
 #include "filter/Filter.hxx"
 #include "lib/ffmpeg/Buffer.hxx"
 #include "lib/ffmpeg/Filter.hxx"
 #include "lib/ffmpeg/Frame.hxx"
 
+#include <cstdint>
+
 /**
  * A #Filter implementation using FFmpeg's libavfilter.
  */
@@ -30,6 +31,11 @@ class FfmpegFilter final : public Filter {
 	const size_t in_audio_frame_size;
 	const size_t out_audio_frame_size;
 
+	/**
+	 * Presentation timestamp.  A counter for `AVFrame::pts`.
+	 */
+	int_least64_t pts = 0;
+
 	bool flushed = false;
 
 public:
@@ -53,5 +59,3 @@ public:
 private:
 	std::span<const std::byte> ReadOutput();
 };
-
-#endif