gfxTextObjectPaint::GetFillPattern/GetStrokePattern should take the destination's CTM
as a parameter in order to set up the pattern matrix correctly, since the pattern matrix
is combined with the CTM by cairo/Thebes --- but we want the pattern rendering to be
independendt of the CTM, instead depending only on the TM set up when we initialized
the gfxTextObjectPaint.
We make SVGTextObjectPaint store matrices that map device space to pattern space.
