backend switching broken

[John W. Eaton]

On 14-Jun-2009, Elias Pipping wrote:

| --------
| Bug report for Octave 3.2.0 configured for x86_64-pc-linux-gnu
| 
| Description:
| -----------
| 
|   * Starting octave, running `backend("fltk")` and invoking plot
|     yields a plot in fltk. Switching to gnuplot using
|     `backend("gnuplot")` works, too -- the next plot will be done by
|     gnuplot -- but switching back does not, i.e. `backend("fltk")` has
|     no effect once gnuplot has been used. 

If I start a new Octave session and execute the commands

  backend ("fltk")
  sombrero
  backend ("gnuplot")
  peaks

both surfaces are drawn in the figure 1 window using the fltk
backend.  If I then do

  figure

the second figure window uses gnuplot.

I'm not sure what the proper behavior is, but it seems OK to me that
the new backend will only apply to new figure windows.  So if you want
to switch, you need to either close existing figure windows or open a
new figure.

Maybe we should issue a warning stating that changing the backend will
only apply to new figure windows?

jwe