function SubImage(from, size, I) {
    if (!I) {
        I = {
            data: null,
            size: size
        };
    }
    this.data = I.data;
    this.originalSize = I.size;
    this.I = I;

    this.from = from;
    this.size = size;
}
Construct representing a part of another {ImageWrapper}. Shares data
between the parent and the child.
@param from {ImageRef} The position where to start the {SubImage} from. (top-left corner)
@param size {ImageRef} The size of the resulting image
@param I {ImageWrapper} The {ImageWrapper} to share from
@returns {SubImage} A shared part of the original image