Fixed RenderTexture being upside down on Android in SFML 2.6 (backport of #2719)

[texus]

This fixes #2419 on the 2.6.x branch

The fix was already merged in the master branch with #2719, this is a backport for the 2.6.x branch

How to test this PR?

The following code is supposed to render a green circle in the top left corner of the screen.
On Android, before the change from this PR is applied, the green circle will appear in the bottom left corner instead.

#include <SFML/Graphics.hpp>
int main()
{
    sf::RenderWindow window(sf::VideoMode::getDesktopMode(), "SFML works!", sf::Style::Fullscreen);

    sf::CircleShape circle(400.f);
    circle.setFillColor(sf::Color::Green);

    sf::RenderTexture texture;
    texture.create(window.getSize().x, window.getSize().y);

    texture.clear();
    texture.draw(circle);
    texture.display();

    sf::Texture copiedTexture = texture.getTexture();
    sf::Sprite sprite(copiedTexture);

    bool active = window.hasFocus();
    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            switch (event.type)
            {
            case sf::Event::Closed:
                window.close();
                break;
            case sf::Event::LostFocus:
                window.setActive(0);
                active = 0;
                break;
            case sf::Event::GainedFocus:
                window.setActive(1);
                active = 1;
                break;
            case sf::Event::KeyReleased:
                if (event.key.code == sf::Keyboard::Escape)
                    window.close();
                break;
            }
        }
        if (!active)
            continue;

        window.clear();
        window.draw(sprite);
        window.display();
    }
}

[ChrisThrasher]

I know you already tested #2719 but can you also please test this on your device? I'd like to be certain that the fix is still valid when applied to this other branch.

[texus]

To be clear, I did retest the issue and solution on the 2.6 branch before making the PR, I didn't just copy the code without checking.
Nevertheless it is always best if someone else can independently verify that it works as well.

[Zombieschannel]

It works, it works, I'm sorry I'm late 😅