Choosing graphics backend for documentation

[Rik]

Ben, Mike,

The scripts that generate the images are m-files in doc/interpreter.  They
all have a subfunction called set_graphics_toolkit() which is shown below.

-- Start of Code --
## This function no longer sets the graphics toolkit; That is now done
## automatically by C++ code which will ordinarily choose 'qt', but might
## choose gnuplot on older systems.  Only a complete lack of plotting is a
## problem.
function set_graphics_toolkit ()
  if (isempty (available_graphics_toolkits ()))
    error ("no graphics toolkit available for plotting");
  endif
endfunction
-- End of Code --

This could be modified to check for the presence of OSMESA and attempt to
use gnuplot if it is not present.

This check works for me

if (octave_config_info ().features.OSMESA)
 ...
endif