Shade triangle-list markers like other objects.

My current project is using triangle-list markers (in interactive markers) to show meshes that are loaded via MoveIt. However the current version of rviz does not do any shading of triangle-list markers.  (It was not computing the surface normals of the faces.)  Therefore triangle-list marker coloring is completely flat, so all you can see is the outline.  This pull request fixes this so that triangle-list markers are shaded the same as meshes and primitive shapes.  I compute the surface normal using the cross-product of the edges.  If the cross-product is (0,0,0), Ogre::Vector3::normalise() does nothing ( http://www.ogre3d.org/docs/api/1.9/_ogre_vector3_8h_source.html#l00446 ), so I don't need to check for it myself.

Before:
![old_box](https://cloud.githubusercontent.com/assets/1562730/5459038/251ace02-850c-11e4-95fb-2ee502119578.png)

After:
![new_box](https://cloud.githubusercontent.com/assets/1562730/5459042/2d035512-850c-11e4-999b-4b0de0919e60.png)