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