crop images #19

I think it should be possible to crop images without data loss. It could work by adjusting the anchor position to take into account the offset caused by cropping. So if we cropped 10 pixels off the left side of the image, move the anchor position 10 pixels to the left.

Note this would require saving the input anchor position, for 2 reasons:

1. So that we could detect modifications to the asset manifest correctly.
2. So that we could generate the original image from the texture