[poppler] Write support

[P. Henrique Silva]

Hi,

Yesterday I started a discussion with krh, tsdgeos on #poppler about
how to add write support on poppler (on the PDF backend).

We have 2 modes of updating a PDF: 1) just update the relevant Object,
update the xref section (with a 'delta' bytes offset) and flush the
file. 2) use incremental update feature[1] of PDF, that just append
Object's to the file and add a new xref and trailer section.

Incremental update looks easier to implement in a first time, but few
things remain a mystery to me:

- how wrappers will use the code?

Every updateable object (annotations, form fields, key signatures, and
so on) will have a poppler object and wrappers will just (duh) wrap
them? or write support should be at a more low-level and must have
only functions to manipulate Object*'s. Letting wrappers to do these
"conversion" from high-level wrapper code to low-level poppler code?

Thanks, and comments are very welcome.

[1] PDF Reference 1.6, Sec. 2.27, 3.3.5

[]'s

-- P. Henrique Silva