Canvas: use weak pointer to protect parent element access.

Using a weak pointer is the best way to ensure no invalid
pointer is used. This also fixes a possible crash in
simgear::canvas::Element::getParentStyle on destructing
canvas elements.

simgear/canvas/elements/CanvasText.cxx

@@ -740,7 +740,7 @@ namespace canvas
   Text::Text( const CanvasWeakPtr& canvas,
               const SGPropertyNode_ptr& node,
               const Style& parent_style,
-              Element* parent ):
+              ElementWeakPtr parent ):
     Element(canvas, node, parent_style, parent),
     _text( new Text::TextOSG(this) )
   {