First patch for surface plots support

[Jean Bréfort <jean brefort normalesup org>]

Hi,

I have initial support for 3D surface plots. Axes are not displayed yet
(much more difficult than the surface rendering itself) as well as a
user interface for the projection settings. I use perspective
projection, but an orthographic projection should also work if the field
of view is set to 0.

I rearranged considerably the plot_surface plugin, so the patch is quite
large (gog_contour_view_render was moved to gog-contour.c). The
compressed is available at
http://jean.brefort.free.fr/surfaces.patch.bz2

Any comments?

Cheers,
Jean