decoder/Client: use std::chrono::duration<double> instead of raw double

src/lib/ffmpeg/Time.hxx

@@ -40,13 +40,13 @@ extern "C" {
  * Convert a FFmpeg time stamp to a floating point value (in seconds).
  */
 gcc_const
-static inline double
+static inline FloatDuration
 FfmpegTimeToDouble(int64_t t, const AVRational time_base) noexcept
 {
 	assert(t != (int64_t)AV_NOPTS_VALUE);
 
-	return (double)av_rescale_q(t, time_base, (AVRational){1, 1024})
-		/ (double)1024;
+	return FloatDuration(av_rescale_q(t, time_base, (AVRational){1, 1024}))
+		/ 1024;
 }
 
 /**