pm3d true depth ordering

[Johannes Z. <joh...@ze...>]

Hello,

I just hacked true depth ordering for pm3d plots. The syntax is

    set pm3d depth

which overrides 'scans(backward|forward|automatic)'.

This is pretty useful for parametric plots for example. You might
want to try it with a parametric torus or the glass.dat example
of the demo directory.

The patch is attached. Should I commit this to CVS?

Comments are welcome.

-- 
Johannes