1999-12-10 Garry R. Osgood <gosgood@idt.net>
	* app/gimpimage.c
	* app/undo.c
        * app/undo_types.h
        * docs/undo.txt
        Changed sanity checks in undo_pop/free_layer_mask() to LAYER_MASK_ADD_UNDO
	and LAYER_MASK_REMOVE_UNDO to be consistent with undo_push_layer_mask()
	These now nvoke proper cleanup and release of GimpLayerMasks.
	* docs/undo.txt: New file, an overview of undo logic written by
	Austin Donnelly
	* app/undo.c
	* app/undo_types.h
	* app/gimpimage.c : Introduced a new UndoType, UNDO_NULL, which maps
	to zero, introducing that value into the enumerated types. Use the
        type to signal type unknown/error/untyped conditions.

	Full patch documentation at
	http://idt.net/~gosgood/gimp-patch/patch02.html