media file: get duration of animated GIFs, PNGs, and ugoiras.

Add methods to MediaFile to calculate the duration, frame count, and
frame rate of animated GIFs, PNGs, Ugoiras, and videos.

Some considerations:

* It's possible to have a GIF or PNG that's technically animated but
  just has one frame. These are treated as non-animated images.

* It's possible to have an animated GIF that has an unspecified
  frame rate. In this case we assume the frame rate is 10 FPS; this is
  browser dependent and may not be correct.

* Animated GIFs, PNGs, and Ugoiras all support variable frame rates.
  Technically, each frame has a separate delay, and the delays can be
  different frame-to-frame. We report only the average frame rate.

* Getting the duration of an APNG is surprisingly hard. Most tools don't
  have good support for APNGs since it's a rare and non-standardized
  format. The best we can do is get the frame count using ExifTool and the
  frame rate using ffprobe, then calculate the duration from that.

app/logical/ffmpeg.rb

@@ -40,27 +40,41 @@ class FFmpeg
   end
 
   def width
-    video_channels.first[:width]
+    video_streams.first[:width]
   end
 
   def height
-    video_channels.first[:height]
+    video_streams.first[:height]
   end
 
   def duration
     metadata.dig(:format, :duration).to_f
   end
 
-  def video_channels
+  def frame_count
+    if video_streams.first.has_key?(:nb_frames)
+      video_streams.first[:nb_frames].to_i
+    else
+      (duration * frame_rate).to_i
+    end
+  end
+
+  def frame_rate
+    rate = video_streams.first[:avg_frame_rate] # "100/57"
+    numerator, denominator = rate.split("/")
+    (numerator.to_f / denominator.to_f)
+  end
+
+  def video_streams
     metadata[:streams].select { |stream| stream[:codec_type] == "video" }
   end
 
-  def audio_channels
+  def audio_streams
     metadata[:streams].select { |stream| stream[:codec_type] == "audio" }
   end
 
   def has_audio?
-    audio_channels.present?
+    audio_streams.present?
   end
 
   def shell!(command)