Better image API

[kfl]

Currently there is no easy way to work with multiple images. For instance, sprites in a game.

I think we need a separate Image type and support API, perhaps something like:

type Image

val dimensions : Image -> {width : int; height : int}
val loadImage : string -> Image
val saveImage : Image -> string -> unit
val drawImage : canvas -> Image -> (int*int) -> unit
val toImage : canvas -> Image