Transparency groups

[otfried]

Support groups with "group opacity" as transparency groups.

This will require a significant rewrite of PdfWriter. Change it so that resources are written out when they are needed (i.e. while writing a page stream, push to write a transparency group into an XForm, then return to the page stream).

This could also solve the problem with #193 if resources are only added to the PDF file when they are actually used.