Gaps in an animation's used frames throws ArrayIndexOutOfBoundsException

Take the frames list of an animated texture and sort it. If there are any gaps between frames in the result, or if the frames do not start at 0, the parser fails with an ArrayIndexOutOfBoundsException. Unused frames at the end of a sprite sheet are fine, but unused ones at the start or in the middle bug out.

For example,

{
	"animation":{
		"frames":[1,2]
	}
}

and

{
	"animation":{
		"frames":[2,0]
	}
}

will fail, but

{
	"animation":{
		"frames":[1,2,0]
	}
}

will succeed.