Allow export of filters allowed by PDF (feBlender ...) as primitives, instead of rasterized

[Andres Gomez (Tanty)]

Inkscape 0.48.5 r10040 for Ubuntu 14.10

Consider:
* https://raw.githubusercontent.com/tanty/design-hodgepodge/master/svg/filter/igalia-feblend-no-grouped-no-group.svg

When exporting to PDF, filter effects are either rasterized or disabled, as explained at:
http://wiki.inkscape.org/wiki/index.php/Release_notes/0.47#PDF.2C_PostScript.2C_and_EPS_export

Some of the filters, including feBlend, have a counterpart in PDF that could be used. For example, the CairoSVG project supports feBlend to be exported as a PDF primitive:
http://cairosvg.org/

The PNG reference look for the SVG is:
https://raw.githubusercontent.com/tanty/design-hodgepodge/master/svg/filter/igalia.png

This is the result exporting with Inkscape and disabling the filters:
https://raw.githubusercontent.com/tanty/design-hodgepodge/master/inkscape/igalia-feblend-no-grouped-no-group--export-ignore-filters.pdf

This is the result using CairoSVG:
https://raw.githubusercontent.com/tanty/design-hodgepodge/master/svg/filter/igalia-feblend-no-grouped-no-group.pdf