dirt propagation infrastructure. The dirt accumulation is happening
	as the graph is manipulated, properties are changed etc automatically.
	The code probably needs more changes as well, but the basics seems
	to work correctly.

	* bin/gegl-projection.c: (gegl_projection_constructor),
	(set_property), (task_render), (task_monitor): Handle dirt
	tracking/cleaning for updates.
	* bin/gegl-view.c: (set_property), (gegl_view_repaint):
	* bin/gegl.c: (main): changed the default project.

	* gegl/Makefile.am: added gegl-{clean,dirt}-visitor.[ch]
	* gegl/gegl-types.h: added dirt and clean visitors.
	* gegl/gegl-dirt-visitor.[ch] NEW:, visitor for propagating dirtied
	regions in the graph.
	* gegl/gegl-clean-visitor.[ch] NEW:, visitor that cleans up dirt
	rectangles.

	* gegl/gegl-node.[ch]: added gegl_node_get_dirty_rect,
	gegl_node_clear_dirt and dirt_rect instance member.
	* gegl/gegl-operation.c: (get_affected_region):

	Added get_affected_region to operations that needed to override
	default behavior.

	* operations/blur/box-blur.c: (get_affected_region), (class_init):
	* operations/blur/gaussian-blur.c: (get_affected_region),
	(class_init):
	* operations/core/shift.c: (get_affected_region), (class_init):
	* operations/transform/affine.c: (op_affine_class_init),
	(get_defined_region), (get_affected_region):
	* operations/generated/porter-duff.rb: (op_affine_class_init).