DecoderAPI: use std::chrono::duration for decoder_seek*()

For type safety and code readability.

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -210,11 +210,19 @@ time_from_ffmpeg(int64_t t, const AVRational time_base)
 		/ (double)1024;
 }
 
+template<typename Ratio>
+static constexpr AVRational
+RatioToAVRational()
+{
+	return { Ratio::num, Ratio::den };
+}
+
 gcc_const
 static int64_t
-time_to_ffmpeg(double t_ms, const AVRational time_base)
+time_to_ffmpeg(SongTime t, const AVRational time_base)
 {
-	return av_rescale_q(t_ms, (AVRational){1, 1000},
+	return av_rescale_q(t.count(),
+			    RatioToAVRational<SongTime::period>(),
 			    time_base);
 }
 
@@ -547,7 +555,7 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 
 		if (cmd == DecoderCommand::SEEK) {
 			int64_t where =
-				time_to_ffmpeg(decoder_seek_where_ms(decoder),
+				time_to_ffmpeg(decoder_seek_time(decoder),
 					       av_stream->time_base) +
 				start_time_fallback(*av_stream);