Image support has been mangled since the move to docbook.css. This
adds a new mediaobject handler to simplify the output HTML so that
images no longer overlap the text and other nearby images.

The output HTML used to look like this :

<div class="MEDIAOBJECT">
  <p><img src="fig2.png"></p>
</div>

The problem was that the image would be displayed on top of nearby
text or image elements completely obscuring the previous contents
rather than doing proper page layout. With this change, the above now
simply looks like this :

<img src="fig2.png">

An alternative solution would be to fix docbook.css, but I think that
the problem stems from the way that different browsers implement CSS.
It is easier to just fix it here at the DSSSL level.