Fixed a bug on not showing images in iOS14 on YYAnimatedImageView

[alisamaiee]

On iOS 13: If we override this, UIKit will render the UIImageView.image itself before calling this method. So even if self.currentFrame == nil, the UIImageView.image get rendered. On iOS 14: UIKit no longer render anything if you override that displayLayer: method. We can have a backup to render layer.contents for this case by ourself.

[JGVB]

Any eta on merge?

[GiuseppePiscopo]

Hi @alisamaiee. Maybe that has already happened, but have you considered submitting this same PR to https://github.com/yykits/yyimage as well? That repository seems still maintained.