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.rb

@@ -135,7 +135,26 @@ class MediaFile
 
   # @return [Boolean] true if the file is animated. Note that GIFs and PNGs may be animated.
   def is_animated?
-    is_video?
+    is_video? || frame_count.to_i > 1
+  end
+
+  # @return [Float, nil] the duration of the video or animation in seconds, or
+  #   nil if not a video or animation, or the duration is unknown.
+  def duration
+    nil
+  end
+
+  # @return [Float, nil] the number of frames in the video or animation, or nil
+  #   if not a video or animation.
+  def frame_count
+    nil
+  end
+
+  # @return [Float, nil] the average frame rate of the video or animation, or
+  #   nil if not a video or animation. Note that GIFs and PNGs can have a
+  #   variable frame rate.
+  def frame_rate
+    nil
   end
 
   # @return [Boolean] true if the file has an audio track. The track may not be audible.
@@ -143,11 +162,6 @@ class MediaFile
     false
   end
 
-  # @return [Float] the duration of the video or animation, in seconds.
-  def duration
-    0.0
-  end
-
   # Return a preview of the file, sized to fit within the given width and
   # height (preserving the aspect ratio).
   #
@@ -179,6 +193,9 @@ class MediaFile
       file_size: file_size,
       md5: md5,
       is_corrupt?: is_corrupt?,
+      duration: duration,
+      frame_count: frame_count,
+      frame_rate: frame_rate,
       metadata: metadata
     }.stringify_keys
   end