small graphics fixes

[John W. Eaton]

On 27-Nov-2009, Shai Ayal wrote:

| I attach a changeset with 3 small changes:
| 1. typo fix
| 2. change rendering order in gl-renderer so that the list of children
| is drawn from end to start -- the first child in the list should  be
| on top of the others
| 3. change colorbar from an image to a surface. while using an image is
| more concise, using a surface has the advantage of working with the
| current gl-renderer, and with gl2ps, so that it will appear in the
| postscript output.
| 
| I plan to begin indexed image support in the gl-renderer (but I'm
| afraid it will still have problems converting to postscript).
| Also, a lot of graphics scripts are not backend-agnostic and contain
| many gnuplot specifics -- most notably. print.m I think they should be
| seperated into backend specific and backend agnostic parts. I will try
| to do this myself, but any help would be appreciated.

I applied the patch to gl-render.h and graphics.cc, but not the change
to the colorbar function.

Switching the colorbar to a surface doesn't work for the gnuplot
backend (see the attached images).  This may be a bug in the gnuplot
backend, but since the colorbar should be an image object for
compatibility with Matlab, I don't think we should change it to a
surface object in Octave.

jwe