Add support for text background area for OpenGL toolkits (bug #39692). * gl-render.h, gl-render.cc (opengl_renderer::draw_text_background): New method. (opengl_renderer::draw_axes_planes) Increase polygon_offset to 9 for axes planes. (opengl_renderer::draw_text) Call draw_text_background prior to drawing text. * gl2ps-print.cc (gl2ps_renderer::draw_text): Ditto. * genpropdoc.m (get_doc): Document relevant text properties. * text.m: Modify second demo to include background properties. * NEWS: Announce new functionality.

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