Decode the file and extract a single image from it. If the file is
animated, the specified frame will be decoded. If there was a problem
decoding the file, null is returned.
Decode a single frame from the data stat was set with startDecode.
If frame is out of the range of available frames, null is returned.
Non animated image files will only have frame 0. An AnimationFrame
is returned, which provides the image, and top-left coordinates of the
image, as animated frames may only occupy a subset of the canvas.