Decouple JellyGifOperation from Foundation.Operation

[frcouceiro]

This PR addresses the following crash:

 Crashed: custom.jelly.gif.animator.queue
 0  libobjc.A.dylib                0x2474 objc_msgSend + 20
 1  Foundation                     0x11fa6c +[__NSOperationInternalObserver _observeValueForKeyPath:ofObject:changeKind:oldValue:newValue:indexes:context:] + 356
 2  Foundation                     0x1def0 -[NSOperation start] + 1196
 3  JellyGif                       0xb764 partial apply for closure #2 in JellyGifAnimator.prepareAnimation() + 4321490788 (<compiler-generated>:4321490788)
 4  JellyGif                       0xaeb8 thunk for @escaping @callee_guaranteed () -> () + 4321488568 (<compiler-generated>:4321488568)

As discussed in https://github.com/TaLinh/JellyGif/issues/8

[BDProject]

This Pr fixed the crash issues that I was having. Please merge in @TaLinh 👍