Hi all!

Atm, feDisplacementmap output depends on zoom factor (see here).
I had a look at the code to try to fix this, but I need someone to validate the code.
Precise meaning of area, unit transforms, and particulraily of the function "enlarge_area" are not perfectly clear to me.

The modifs are all in nr-filter-displacement-map.cpp.
I just changed "scale" with "scalex" and "scaley" defined as:

    Matrix trans = units.get_matrix_primitiveunits2pb();
    double scalex = scale*trans.expansionX();
    double scaley = scale*trans.expansionY();

where apparently appropriate (in the render() and enlarge_area() methods), but I would feel more confortable if someone having a better understanding of the code could check if this is correct or if other changes are needed elsewhere...

Thanks!
jfb.