LibWeb: Treate SVG paintable coordinates as relative to <svg> element

Normally, paintable coordinates are relative to the nearest containing
block, but in the SVG case, since <svg> doesn't form a containing block,
we have to specialize the computation of SVGPaintable::absolute_rect().

Userland/Libraries/LibWeb/Painting/SVGPaintable.h

@@ -20,6 +20,8 @@ public:
 
 protected:
     SVGPaintable(Layout::SVGBox const&);
+
+    virtual Gfx::FloatRect compute_absolute_rect() const override;
 };
 
 }