Initial implementation of a LaTeX interpreter (bug #59546). * NEWS: Announce basic support for "latex" interpreter. * plot.txi: Restructure the "Use of the interpreter Property" section to include 3 subsections, one for each interpreter type. In "Printing and Saving Plots" describe the new behavior. * print.m: Document new behavior. * contributors.in: Add Andrej Lojdl, a former GSoC student that worked on this subject. Even though there is not much left of his original work, it served as a very useful starting point. * base-text-renderer.h, base-text-renderer.cc: New enum to store symbolic constants for rotation angles. (base_text_renderer::rotate_pixels): New utility method. Code extracted from ft_text_renderer::render. (base_text_renderer::rotation_to_mode): Moved from ft_text_renderer class. (base_text_renderer::fix_bbox_anchor): New utility method. Code extracted from ft_text_renderer::text_to_pixels. * ft_text_renderer.cc (ft_text_renderer::render, ft_text_renderer::text_to_pixels): Make use of base class utility functions. * gl2ps-print.cc: New counter variable m_svg_def_index for safe inclusion of defs coming from dvisvgm. (gl2ps_renderer::format_svg_element): New function to manipulate svg elements obtained from dvisvgm so as to position them properly on the figure and change their color. (gl2ps_renderer::strlist_to_svg): If the str_list object returned from text_to_strlist contains an svg element, use format_svg_element to handle position and color and then return it. Otherwise, use <g> and <text> elements rather than <text> and <tspan> elements which are not supported by Qt's svg renderer (svg-tiny implementation). (gl2ps_renderer::strlist_to_ps): If the str_list object returned from text_to_strlist contains an svg element, raise a warning about the necessity of using -svgconvert. * latex-text-renderer.h, latex-text-renderer.cc: New files to hold the latex_interpreter class. * libinterp/corefcn/module.mk: Add new files to the build system. * text-renderer.h, text-renderer.cc (text_renderer): New data member latex_rep to hold a pointer to an instance of a latex_renderer. (text_renderer::~text_renderer, get_extent, set_anti_aliasing, set_font, set_color, text_to_pixels, text_to_strlist): Duplicate action of the latex_rep. (text_renderer::latex_ok): New method to test the usability of the latex_renderer. (text_renderer::string::svg_element, text_renderer::string::set_svg_element, text_renderer::string::get_svg_element): New string data member to hold preformated svg element. Provide accessor methods. * acinclude.m4: Add QtSvg to the list of imported QT_MODULES. * octave-svgconvert.cc: Overhaul program to make use of Qt's QSvgRenderer when rendering to PDF.

--- a/libinterp/corefcn/text-engine.h	Mon Mar 29 07:54:26 2021 +0200
+++ b/libinterp/corefcn/text-engine.h	Mon Mar 22 21:32:54 2021 +0100
@@ -322,7 +322,7 @@
   text_processor
   {
   public:
-    virtual void visit (text_element_string& e) = 0;
+    virtual void visit (text_element_string&) { }
 
     virtual void visit (text_element_symbol&) { }