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