gdip: fix gif animation delays between frames

1. delay value of first frame was being used for all frames
2. when a frame specifies a delay of 0ms, use 100ms instead of 20ms
   (matches gdk-pixbuf/io-gif.c)
3. when a frame specifies a delay of < 20ms, use 20ms instead
   (also matches gdk-pixbuf/io-gif.c)

https://bugzilla.gnome.org/show_bug.cgi?id=655755