- cleaned up do_merge() and got rid of some useless variables always set to 0
- added options for outside-pixel mask creation and cropping to rectangle to
  the GUI
- actually implemented the first one


git-svn-id: svn://svn.code.sf.net/p/gwyddion/code/trunk/gwyddion@16825 25697fe4-d644-49b8-9d30-be5465f2adf3