Proper fix for multipage PDF+LaTeX export

While working on !133 (merged) I noticed the previous attempt (!13 (merged)) to fix the issue with missing pages in the PDF+LaTeX export [1] was only outputting blank pages and usually produced one PDF page with all output plus a lot of empty pages therefore defeating the original idea of having text being overlayed by transparent objects [2].

In this MR I reverted this workaround and tried to replace it with a "proper" fix that actually outputs partial renderings of the final document which can then be stacked in the LaTeX document with the text sandwiched in between.

As I'm just becoming acquainted with our Cairo renderer and I'm not familiar with Cairo itself either this needs a lot of testing! The samples.zip I've thrown at it were converted properly but I'm pretty sure I didn't think of all the possible border cases, so please check with your own files and hope that the new renderer hack does not choke on them 😉

[1] https://bugs.launchpad.net/inkscape/+bug/1417470
[2] https://bugs.launchpad.net/inkscape/+bug/771957