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/media_file/ugoira.rb

@@ -32,6 +32,22 @@ class MediaFile::Ugoira < MediaFile
     preview_frame.crop(width, height)
   end
 
+  def duration
+    (frame_delays.sum / 1000.0)
+  end
+
+  def frame_count
+    frame_data.count
+  end
+
+  def frame_rate
+    frame_count / duration
+  end
+
+  def frame_delays
+    frame_data.map { |frame| frame["delay"] }
+  end
+
   # Convert a ugoira to a webm.
   # XXX should take width and height and resize image
   def convert