#1072 epub: Unreferenced callout icons in OPF

[Stuart Rackham]

When callouts are used in a document and callout graphics are disabled
(callout.graphics=0) the generated 'contents.opf' still contains
references to all the callout icons even though none are not
referenced in the generated XHTML content. This results in 10
epubcheck validation errors like:

image file OEBPS/images/icons/callouts/1.png is missing

It appears that epub is adding the icons to the OPF without
first checking the callout.graphics parameter.

Environment: DocBook XSL 1.75.2; Ubuntu 8.04