[docbook-css] Different image resources for HTML & FO output

[martin.gautier@myrnham.co.uk]

Hiya

The DocBook stylesheets have hard-coded support for the "role" attribute 
in the <imageobject> element such that authors can include multiple 
<imageobject>s with their own roles of "html" or "fo". This allows 
different image resolutions depending on the target. The stylesheets are 
built to support this. Here's an example:

<inlinemediaobject>
  <imageobject role="html"><imagedata format="PNG" 
fileref="figure\dialog_100.png"/></imageobject>
  <imageobject role="fo"><imagedata format="PNG" 
fileref="figure\dialog_300.png"/></imageobject>
</inlinemediaobject>

When viewing the XML through a browser with docbook-css you might not want 
to see both images. My customisation layer now includes:

imageobject[role="fo"] {
  display: none;
}

which avoids the problem.

Mart