media assets: save audio volume levels in media metadata.

For videos with sound, save information about audio volume levels in the
media asset's metadata. These values are stored:

* FFmpeg:AudioPeakLoudness       The peak loudness of the audio track, from 0.0 (silent) to 1.0 (max volume)
* FFmpeg:AudioAverageLoudness    The average loudness of the audio track, from 0.0 (silent) to 1.0 (max volume).
* FFmpeg:AudioLoudnessRange      The difference between the quietest and loudest sounds in the audio track (in decibels).
* FFmpeg:AudioSilencePercentage  The percentage of the video that is silent (1.0 is completely silent, 0.5 is 50% silence, 0.0 is no silence).

These values are calculated based on the EBU R 128 standard, using the ffmpeg command below:

  ffmpeg -i file.mp4 -af silencedetect=duration=0.05:noise=0.0001,ebur128=metadata=1:peak=true:dualmono=true -f null /dev/null

See the links below for details:

* https://en.wikipedia.org/wiki/EBU_R_128
* https://www.ffmpeg.org/ffmpeg-filters.html#ebur128-1
* https://tech.ebu.ch/loudness
* https://tech.ebu.ch/docs/tech/tech3341.pdf

app/logical/media_file/video.rb

@@ -5,7 +5,10 @@
 #
 # @see https://github.com/streamio/streamio-ffmpeg
 class MediaFile::Video < MediaFile
-  delegate :duration, :frame_count, :frame_rate, :has_audio?, :is_corrupt?, :major_brand, :pix_fmt, :video_codec, :video_bit_rate, :video_stream, :video_streams, :audio_codec, :audio_bit_rate, :audio_stream, :audio_streams, :error, to: :video
+  delegate :duration, :frame_count, :frame_rate, :has_audio?, :is_corrupt?, :major_brand, :pix_fmt,
+    :video_codec, :video_bit_rate, :video_stream, :video_streams, :audio_codec, :audio_bit_rate,
+    :audio_stream, :audio_streams, :silence_duration, :silence_percentage, :average_loudness,
+    :peak_loudness, :loudness_range, :error, to: :video
 
   def dimensions
     [video.width, video.height]
@@ -28,6 +31,10 @@ class MediaFile::Video < MediaFile
       "FFmpeg:AudioProfile" => audio_stream[:profile],
       "FFmpeg:AudioLayout" => audio_stream[:channel_layout],
       "FFmpeg:AudioBitRate" => audio_bit_rate,
+      "FFmpeg:AudioPeakLoudness" => peak_loudness,
+      "FFmpeg:AudioAverageLoudness" => average_loudness,
+      "FFmpeg:AudioLoudnessRange" => loudness_range,
+      "FFmpeg:AudioSilencePercentage" => silence_percentage,
     }.compact_blank)
   end