Resize HImage's internal _image.

The internal _image (of type PImage) must be resized because certain HYPE funcationality depends on this internal image. An example is HPixelColorist which uses PImage's `get` function to retrieve the pixel color information. Therefore, without resizing _image, if you pass an HImage that has been resized into HPixelColorist, the pixel colors are retrieved from the original size and not the new size.