posts: fix calculation of animated PNG duration.

Fix certain animated PNGs returning NaN as the duration because the
frame rate was being reported as "0/0" by FFMpeg. This happens when the
animation has zero delay between frames. This is supposed to mean a PNG
with an infinitely fast frame rate, but in practice browsers limit it to
around 10FPS. The exact frame rate browsers will use is unknown and
implementation defined.

app/logical/media_file/image.rb

@@ -61,8 +61,9 @@ class MediaFile::Image < MediaFile
     if is_animated_gif?
       frame_count / duration
     elsif is_animated_png?
-      # XXX we have to resort to ffprobe to get the frame rate because libvips and exiftool can't get it.
-      video.frame_rate
+      # 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