Add Image methods to allow stretching

[pcantrell]

Problem

It’s currently not possible to stretch an Image beyond the source image file’s dimensions — to fill the background of a window, for example.

Desired solution

One possible solution would be to add an explicit setSize() method that stretches the image regardless of its natural size.

Another would be to provide an option for the max width/height params to switch from their current “shrink to fit” effect to “stretch to fill.”