Changes

`num_iterations` indicates the number of iterations that this animation should play; if it is 0, the animation should play indefinitely. If nonzero, the animation should come to rest on the final non-skipped frame at the end of the last iteration.

The `fCTL` chunk is an ancillary chunk as defined in the PNG Specification. It must appear before the IDAT chunks of the frame to which it applies, specifically: