[Graphics] Add utility functions

[John W. Eaton]

On 30-Jan-2008, Michael Goffioul wrote:

| OK, here's a more usable solution.
| 
| Michael.
| 
| 
| 2008-01-30 Michael Goffioul <michael.goffioul at gmail.com>
| 
|     * graphics.h.in (axes::properties::get_transform_matrix,
|     axes::properties::get_inverse_transform_matrix,
|     axes::properties::get_opengl_matrix_1,
|     axes::properties::get_opengl_matrix_2,
|     axes::properties::get_transform_zlim): New accessors.
|     (base_properties::is_clipping): New accessor.
|     (class graphics_xform): New class encapsulating
|     axes transformation.
|     (axes::properties::get_transform): New method returning
|     a graphics_xform object.
|     * graphics.cc (class graphics_xform): New class.

I applied this patch and checked in the changes.

Thanks,

jwe