derivatives getting built for every view of larger-than-original sizes

Here's what's happening:

Upon uploading a new image, image.module runs the _image_build_derivatives() function to create the various sizes. However, if one of the sizes is *larger* than the original size, it does not create a file for this size, it just substitutes the _original information for whatever label represents the larger size.

The problem is that when anyone goes to view this larger size again, image.module runs the entire _image_build_derivatives() function and recreates all of the sizes. This blows away image_exact.module images and is very inefficient.

This patch stores the original image as the image for the larger size so that the derivatives aren't built again upon viewing of this size.