Fix missing legend background in gl2ps outputs (bug #44765) * gl2ps-render.h (glps_renderer): new private attribute "naxes" used to count drawn axes * gl2ps-renderer.h (glps_renderer::draw_axes): new overloaded method. Use gl2psBegin(End) viewport to Finalize/Initialize a new viewport each time an axes is drawn. This is supposed to mimic "glClear (... GL_DEPTH_BUFFER_BIT)" in opengl_renderer::draw_axes

# fA.m
function y = fA (x, f)
  global gfun
  if nargin < 2
    y = fA (x, gfun);
  else
    w = feval (f, x);
    y = feval (@fB, w);
  endif
endfunction