Adds copying to C4Image

[traviskirton]

class ViewController: C4CanvasController {
var img = C4Image("chop")!

override func setup() {
for _ in 0...3 {
guard let i = img.mutableCopy() as? C4Image else {
continue
}
i.center = C4Point(random01() * canvas.width, random01() *
canvas.height)
canvas.add(i)
}
}
}

[traviskirton]

@Aleph7: added constructor, but i'm not a fan of:

C4Image(c4image: c!)

That's just bad grammar. But, C4Image(_ c4image: c!) conflicts with: C4Image(_ name: String)

[alejandro-isaza]

I think it should be C4Image(named: String) same as UIImage, but that breaks the API. We could also do let newImage = image.imageByCopying()

[traviskirton]

what about

img.c4image

this is consistent with img.cgimage, img.ciimage, img.uiimage