Make it easier to set watermark for PDF output

[raducoravu]

For Docbook for example you have a parameter which controls this:

http://docbook.sourceforge.net/release/xsl/1.75.2/doc/fo/draft.watermark.image.html

For DITA you would need to overwrite in your custom.xsl a lot of templates, starting with the one which generates the frontmatter + many templates from the "org.dita.pdf2/xsl/fo/static-content.xsl" like "static-content.xsl".
At least we would have a single XSLT template which we could overwrite for this in a customization.

Looking also at Docbook, they have lots of parameters for PDF customization. This happens because in Docbook it's very easy to define a parameter, you just define it as an XSLT parameter.
For DITA you always need to also define an ANT parameter which tunnels its value to the XSLT parameter.

[stale]

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