Preserve previous crop

[howff]

If the new image dimensions match the previous image then set the initial crop rectangle to be the same as the previous

[howff]

Sorry I mixed up several fixes into the same PR.

1. Two commits are to add the feature requested in Feature Wish: Cropping, without resetting crop-selection to full image size #59
2. One commit fixes the magick command which needs +repage otherwise the cropped image cannot be further cropped
3. Only use the 'f' suffix when cropping JPEG format

[ConradHughes]

Looks good — sorry for the ridiculously long wait.