Adding support for animated gifs

I added support to gifs. I'm sorry but there is no way to achieve animated gifs without adding two functions to graphics-ffmpeg.c  since the original functions only supported one image while mine support a pointer to multiple images. i named them like the originals but adding _gif at the end.
To use them, you just pick "image" from sources and set "Animate Gifs". After all it's pretty self explanatory.