Add the ability to render static gifs

[Havhingstor]

The ImageView now tests if a gif is an animated gif, so that the animator only tries to animate it, if necessary. A static gif is rendered like any other image.

Previously, it was tried to animate every gif, whether it truly was animated or not. That resulted in no image being rendered at all.

[kean]

Thanks, @Havhingstor. There were a couple of related issues reports, but nobody got to the bottom of this until now.

By the way, I'm considering removing the GIF support from the main framework entirely and directing users to integrate Gifu directly. The integration should only require a single line.

I think it's great to handle this scenario on the ImageView level for now. But I'd suggest also opening a similar PR in Gifu if you have time. I think it should be able to handle this scenario automatically.

Update: might be related kaishin/Gifu#174

[kean]

I'm wondering how expensive CGImageSourceCreateWithData is going to be? Presumably not too expensive because it's not going to eagerly bitmap the image unless instructed to do so.

[Havhingstor]

Thanks!