Add ability to save animated GIF

[Xrayez]

This adds ability to save both static and animated GIFs.

Example

This rotating Goost icon was produced with:

func save_gif_animated_rotation():
	var image_frames = ImageFrames.new()

	var angle = 0.0
	for i in 32:
		var frame = Image.new()
		frame.load("res://icon.png")
		GoostImage.rotate(frame, angle, false)
		var delay = 0.02
		image_frames.add_frame(frame, delay)
		angle += TAU / 32.0

	image_frames.save_gif("res://animated_rotation.gif")