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

NEWS

@@ -1,6 +1,8 @@
 ver 0.23.16 (not yet released)
 * database
   - fix integer overflows with 64-bit inode numbers
+* filter
+  - ffmpeg: fix for filters producing no output
 * support libfmt 11
 * support ICU 76
 

src/filter/plugins/FfmpegFilter.cxx

@@ -66,6 +66,9 @@ FfmpegFilter::FilterPCM(ConstBuffer<void> 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

@@ -17,14 +17,15 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#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.
  */
@@ -46,6 +47,11 @@ 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;
+
 public:
 	/**
 	 * @param _graph a checked and configured AVFilterGraph
@@ -63,5 +69,3 @@ public:
 	/* virtual methods from class Filter */
 	ConstBuffer<void> FilterPCM(ConstBuffer<void> src) override;
 };
-
-#endif