New UIImage kind of object to cache offscreen images/textures

We should be able to create an `UIImage()` from an `Image` (and maybe from scratch or other objects), and getting a `GraphicsContext` from it so we can paint on it as an offscreen `Canvas` object. The `UIImage` should be able to be the fastest representation to paint an image on the screen.

E.g.
```lua
local img = UIImage(app.activeImage)
Dialog():canvas{ ...
onpaint=function(ev)
local ctx = ev.context
ctx:drawImage(img, 0, 0)
end
}
```