Support other image formats

[kean]

I suggest adding an option to extend the framework to add other image decoders, e.g. WebP, APNG. Most of the display logic can be presumably reused across the formats.

There were attempts to extend FLAnimatedImage to support additional formats, but none of them got merged. I don't know if there exists a framework that does these things without also bundling a massive image loading and caching library with it.

[eliburke]

Core Image supports GIF natively. That's the display logic. Gifu just makes it play nice with Swift.