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.
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.
Generate smart previews as .png so we don't suffer recompression losses
when we convert the preview frame from a full size image down to a
150x150 .jpg thumbnail.
