LibWeb: Try to work out the intrinsic size of <svg> elements

If the `width` and `height` attributes are provided, we derive the
intrinsic size and ratio from them.

Otherwise, we trace a rectangle around the geometry elements inside
the SVG and use the size of that as the intrinsic size.

This is definitely far from correct, but is still a much better guess
at the intrinsic size than nothing.

Userland/Libraries/LibWeb/Layout/SVGSVGBox.h

@@ -21,6 +21,8 @@ public:
     virtual bool can_have_children() const override { return true; }
 
     virtual RefPtr<Painting::Paintable> create_paintable() const override;
+
+    virtual void prepare_for_replaced_layout() override;
 };
 
 }