Incorrect glyph position with text outline

[cschreib]

Subject of the issue

I am using SFML as one of multiple back-ends for a GUI library. I have experienced some issues with the outline glyph positions returned by sf::Font, see the : and + glyphs on the screenshot below (from current master):

Your environment

• Your OS / distro / window manager used: Linux Mint 20.2 Cinnamon
• Your version of SFML (2.5.0, git master, etc): master
• Your compiler and compiler version used: GCC 9.3 and Clang 10 or 11
• Special compiler flags used: none

Steps to reproduce

Tell us how to reproduce this issue. Please provide a minimal, complete and verifiable example, you can use the follow template as a start:

• Font I use for testing: noto.ttf.zip
• Code:

#include <SFML/Graphics.hpp>
#include <cmath>

int main() {
    sf::RenderWindow window(sf::VideoMode(768, 512), "Minimal, complete and verifiable example");
    window.setFramerateLimit(60);

    sf::Font fnt;
    fnt.loadFromFile("noto.ttf");

    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            if (event.type == sf::Event::Closed)
                window.close();
        }

        window.clear(sf::Color(128, 128, 128));

        float y = 0.0;
        for (std::size_t i = 5; i < 20; ++i) {
            sf::Text txt("Language: C++", fnt, i);
            txt.setFillColor(sf::Color::White);
            txt.setOutlineColor(sf::Color::Black);
            txt.setOutlineThickness(2.0f);
            txt.setPosition(0.0, std::round(y));
            window.draw(txt);

            y += 1.5f*i;
        }

        y = 0.0;
        for (std::size_t i = 5; i < 20; ++i) {
            sf::Text txt("Language: C++", fnt, i);
            txt.setFillColor(sf::Color::White);
            txt.setStyle(sf::Text::Bold);
            txt.setOutlineColor(sf::Color::Black);
            txt.setOutlineThickness(2.0f);
            txt.setPosition(256.0, std::round(y));
            window.draw(txt);

            y += 1.5f*i;
        }

        y = 0.0;
        for (std::size_t i = 5; i < 20; ++i) {
            sf::Text txt("Language: C++", fnt, i);
            txt.setFillColor(sf::Color::White);
            txt.setStyle(sf::Text::Italic);
            txt.setOutlineColor(sf::Color::Black);
            txt.setOutlineThickness(2.0f);
            txt.setPosition(512.0, std::round(y));
            window.draw(txt);

            y += 1.5f*i;
        }

        window.display();
    }
}

Expected behavior

The outline (black) for the : and + glyphs should be rendered well centred behind the non-outline (white) glyph.

Actual behavior

They are not.

[cschreib]

I have a proposed fix in #1827.