- finished mask creation and cropping implementation, also for non-correlation
  merges
- got rid of all the ugly code calculating mutual field positions after
  correlation search and replaced it with a simple calculation


git-svn-id: svn://svn.code.sf.net/p/gwyddion/code/trunk/gwyddion@16826 25697fe4-d644-49b8-9d30-be5465f2adf3