We left it out, not because it is difficult, but because we didn't want somebody coming from graphic design to make the choice to stretch to fit not realizing that fit to box would accomplish what they need without throwing off proportions.
We couldn't think of a use case where someone would actually want the image stretched. So we left it out so that you wouldn't choose it by accident because you didn't understand fit to box will work for you.
In a way we left this feature out because of you, and it would appear to me it would have hurt you if it were there because you may have used it when fit to box will actually work better for your application.
