Separation of responsibility between rendering and window

[rtkid-nt]

In the current state, the window and rendering are tightly coupled, and you can't use anything trivial other than OpenGL to draw
I know about WindowBase::createVulkanSurface, but to use it, you must write self rendering code.

What do you think about:

1. Sharing responsibility between rendering and window?
2. Making the option to choose api for drawing: Vulkan, OpenGL, DirectX

sf::Renderer - some static class representing renderer
sf::Renderer::API - enum or constexpr variable representing available api

class ShaderImpl
{
public:
    virtual ~ShaderImpl() = default;
    virtual void setUniform(const std::string& name, float x) = 0;
};

class Shader
{
public:
    Shader() 
    {
        switch (sf::Renderer::getAPI())
	{
	    case sf::Renderer::API::OpenGL: m_impl = new sf::priv::ShaderImplOpenGL; break;
	    case sf::Renderer::API::Vulkan: m_impl = new sf::priv::ShaderImplVulkan; break;
            case sf::Renderer::API::DirectX: m_impl = new sf::priv::ShaderImplDirectX; break;
	}
    }

    ~Shader()
    {
        delete m_impl;
    }

    virtual void setUniform(const std::string& name, float x)
    {
	m_impl->setUniform(name, x);
    }

private:
    ShaderImpl* m_impl{};
};

class ShaderImplOpenGL final : public ShaderImpl
{
public:
    void setUniform(const std::string& name, float x) override {}
};

class ShaderImplVulkan final : public ShaderImpl
{
public:
    void setUniform(const std::string& name, float x) override {}
};

class ShaderImplDirectX final : public ShaderImpl
{
public:
    void setUniform(const std::string& name, float x) override {}
};

[eXpl0it3r]

This will certainly be a topic when addressing #2423, as such I'll currently close this as "duplicate".