Add Min and Max blend modes

[Sakarah]

• Has this change been discussed on the forum or in an issue before?
• Does the code follow the SFML Code Style Guide?
• Have you provided some example/test code for your changes?

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Description

This PR adds two blend modes that are necessary for some effects, namely BlendMin and BlendMax.
Given the fact that constructing Min and Max from factors and equation might seem weird (because factors are ignored), I chose to provide constants.
It supports both OpenGL and OpenGL ES with the appropriate extension GL_EXT_blend_minmax.
It should also allow implementations with GL_EXT_blend_subtract but without GL_EXT_blend_minmax to use BlendMode::Subtract and BlendMode::ReverseSubtract.

This PR implements the suggestion #1710.

Tasks

• Tested on Linux
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

How to test this PR?

Here is a simple example to test the added blending modes. Place two images 1.png and 2.png in the working directory. Then press A to see them max blended, and Z to see them min blended.

#include <SFML/Graphics.hpp>

int main()
{
    sf::RenderWindow window;
    window.create(sf::VideoMode(128,128), "Min/Max blending test", sf::Style::Default);

    sf::Texture texture1;
    texture1.loadFromFile("1.png");
    texture1.setSmooth(true);
    sf::Sprite sprite1;
    sprite1.setTexture(texture1);

    sf::Texture texture2;
    texture2.loadFromFile("2.png");
    texture2.setSmooth(true);
    sf::Sprite sprite2;
    sprite2.setTexture(texture2);

    sf::Color clearColor = sf::Color::Black;
    sf::RenderStates states;
    states.blendMode = sf::BlendMax;

    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            if (event.type == sf::Event::Closed || (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Key::Escape))
            {
                window.close();
            }
            else if (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Key::A)
            {
                states.blendMode = sf::BlendMax;
                clearColor = sf::Color::Black;
            }
            else if (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Key::Z)
            {
                states.blendMode = sf::BlendMin;
                clearColor = sf::Color::White;
            }
        }

        window.clear(clearColor);
        window.draw(sprite1, states);
        window.draw(sprite2, states);
        window.display();
    }

    return 0;
}

[eXpl0it3r]

Tested it locally, it works and looks correct.

Thank you for your contribution! 🙂