media file: fix calculation of video/animation duration.

Fix how the duration of videos and animated GIFs / PNGs is calculated.
If we can't determine the duration from the file metadata, then play the
entire video or animation back using FFmpeg and scrape the duration and
frame count.

This is necessary for things like WebM files where the duration metadata
is optional, or animated GIFs and PNGs that don't have a duration field
in the metadata, only a frame count and a sequence of frame delays.

app/logical/media_file/image.rb

@@ -24,27 +24,8 @@ class MediaFile::Image < MediaFile
   end
 
   def duration
-    if is_animated_gif?
-      if metadata.has_key?("GIF:Duration")
-        # GIF:Duration => "9.03 s"
-        metadata["GIF:Duration"].to_f
-      else
-        # If GIF:Duration is absent then it means the GIF has an unlimited
-        # framerate. In this situation we assume the browser will play the GIF
-        # at 10 FPS; this is browser dependent.
-        #
-        # A GIF consists of a series of frames, each frame having a separate frame
-        # delay. The duration of the GIF is the sum of each frame delay. If the frame
-        # delay of each frame is zero, then it means the GIF has an unlimited framerate
-        # and should be played as fast as possible. In reality, browsers cap the
-        # framerate to around 10FPS.
-        (frame_count * (1.0/10.0)).round(2)
-      end
-    elsif is_animated_png?
-      frame_count.to_f / frame_rate
-    else
-      nil
-    end
+    return nil if !is_animated?
+    video.duration
   end
 
   def frame_count
@@ -58,15 +39,8 @@ class MediaFile::Image < MediaFile
   end
 
   def frame_rate
-    if is_animated_gif?
-      frame_count / duration
-    elsif is_animated_png?
-      # XXX As with GIFs, animated PNGs can have an unspecified frame rate.
-      # Assume 10FPS if the frame rate is unspecified.
-      video.frame_rate.presence || 10.0
-    else
-      nil
-    end
+    return nil if !is_animated? || frame_count.nil? || duration.nil? || duration == 0
+    frame_count / duration
   end
 
   def channels
@@ -103,6 +77,10 @@ class MediaFile::Image < MediaFile
     end
   end
 
+  def is_animated?
+    frame_count.to_i > 1
+  end
+
   def is_animated_gif?
     file_ext == :gif && is_animated?
   end