vertical-motion bug

[Chong Yidong <cyd@stupidchicken.com>]

In a scratch buffer:

  M-<
  M-: (insert (propertize "a" 'display "a\nb\nc\n")) RET
  M-<
  C-n (or M-: (vertical-motion 1))

Result: point moves down three lines.  The expected behavior, based on
the `vertical-motion' docstring, is to move just one line.  This
behavior was broken by something in the following change:

2006-06-14  Kim F. Storm  <storm@cua.dk>

	* dispextern.h (IT_STACK_SIZE): New macro specifying size of
	iterator stack (instead of hardcoded number).  Increase from 2 to
	4 to make room for propertized overlay strings before and after a
	display string, image or composition.
	(struct it): Add image_id and method members to iterator stack.

	* xdisp.c (init_from_display_pos): Don't set it->method and
	overlay_string_index after pop_it.  Add asserts.
	(handle_stop): Look for overlay strings around a display string,
	image, or composition.  Handle properties on those strings.
	(next_overlay_string): Don't set string, pos or method after pop_it.
	(get_overlay_strings_1): Split from get_overlay_strings; don't
	modify it if no overlay strings are found.
	(get_overlay_strings): Use get_overlay_strings_1.  Always set
	it->string and it->method.
	(push_it): Push it->image_id and it->method.  Push it->object
	instead of it->string if method is GET_FROM_IMAGE.
	(pop_it): Pop it->image_id and it->method.  Ppo it->object
	instead of it->string if method is GET_FROM_IMAGE.
	Reset it->current.string_pos if popped it->string is nil.
	(reseat_1): Remove comment dated 19 May 2003.  It expressed doubt
	whether a given change was correct; but the change is correct.
	Clear it->string_from_display_prop_p.
	(set_iterator_to_next): Rely on it->method and it->image_id from
	iterator stack, instead of setting them explicitly after pop_it.