improve vertical positioning of `@image`

[Werner LEMBERG]

Right now, at the top of a page, the height of an `@image` box is
aligned with the base line of the top line.  I ask to shift the image
box up so that it aligns with the top of the top (default text) line,
since the current behaviour is not optimal for two reasons.

* If there are many images mixed with text, the top of the text block
  is vertically jumping a lot.

* It is a waste of space.  For example, LilyPond's notation reference
  has more than 900 pages including almost 1700 images; a raised box
  would save a few pages.


    Werner