Combines a set of Attribute<!-- -->s with a specific draw mode and 
            defines a vertex count so a Primitive object can be retained and drawn later with no 
            addition information required.
The value passed as n_vertices will simply update the Primitive <
            structfield>n_vertices</structfield> property as if set_n_vertices
             were called. This property defines the number of vertices to read when drawing.
| mode | A VerticesMode defining how to draw the vertices | 
| n_vertices | The number of vertices to process when drawing | 
| ... | A null terminated list of attributes | 
| A newly allocated Primitive object |