PDF generator sets transparency to black

[robe]

I think this is an issue I mentioned before and for the wkt raster generator, Kevin had just changed it to change transparency to white.

It's only a minor inconvenience, but would be nice if we nipped it at the main issue.

For example I've been using the ST_AsPNG function in raster to output images like the one seen here:  http://www.postgis.org/documentation/manual-svn/RT_ST_Clip.html

Looks great on the html docs and the PNGs generated have correct transparency which I can see when I open it in Paint.net for example. However I look at the generated PDF and yuck its replaced the beautiful transparency with black so have to go in and make the transparency white.