Use the presentation name/page number to match shapes to pages, not time.

The previous code assumed that if a shape event's timestamp was during the
time when a particular slide was visible, then the shape belonged to that
slide. This isn't always true with the text shapes, since a final close
event can be sent after the slide has switched.

For a more future-proof matching, use the presentation name and page number
fields on the shape events when available to match them with the appropriate
slide image.