2004-12-23  Michael Natterer  <mitch@gimp.org>

	Item conversion depends on the old *and* the new item type, so
	it can't live in the old item's vtable only:

	* app/core/gimpitem.[ch]: split GimpItem::convert() into
	GimpItem::convert_from() (which is called on the old item and
	creates the new item) and GimpItem::convert_to() (which is called
	on the new item). This way functions from the old *and* new items'
	vtables are called and it's possible to convert between item types
	which live on different branches of the class hierarchy or to item
	types which live further down the class tree than the old item.

	(gimp_item_convert): call ::convert_to() on the new item created
	by ::convert_from().

	* app/vectors/gimpvectors.c: changed ::convert() implementation
	to ::convert_from().

	* app/core/gimplayer.c: changed ::convert() to ::convert_to().
	Fixes bug #161877.