Fix Layer_PasteCanvas.
    
    Layer_PasteCanvas::get_bounding_rect() requires ContextParams to
    calculate size of content.
    
    So
    
    get_bounding_rect() - now always returns empty rectangle
    
    but
    get_full_bounding_rect() - works fine, because this function knows about
    Context.
    
    also for Layer_PasteCanvas implemented additional method
    get_bounding_rect_context_dependent() which also work fine, because
    requires context_params
    
    get_bounding_rect() method uses in duckMatick::add_ducks_layers() only,
    so this method should be improved to calculate valid size of
    Layer_PasteCanvas