Separate layer visibility from feature visibility.

A single commit to separately control feature and layer visibility. A given feature is only displayed if both its visibility flag **and** its layer's visibility are true. I *think* this is the most intuitive logic.
I got distracted earlier, Aashish, but I just pushed in the commit.

On Fri, Nov 7, 2014 at 3:26 PM, Aashish Chaudhary <notifications@github.com>
wrote:

> In vtkMapTile.cxx:
>
> > @@ -230,7 +230,8 @@ void vtkMapTile::CleanUp()
> >  //----------------------------------------------------------------------------
> >  void vtkMapTile::Update()
> >  {
> > -  this->Actor->SetVisibility(this->Visibility);
> > +  // Visible only if both layer and feature visibility flags are set
> > +  bool visibility = this->GetLayer()->GetVisibility() && this->Visibility;
> > +  this->Actor->SetVisibility(visibility);
>
> this->Actor->SetVisibility(this->IsVisible());
>
> .....
>
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> <https://github.com/OpenGeoscience/vtkMap/pull/58/files#r20035096>.
>