Add color blending modes to sf::BlendMode

[FRex]

So the idea of 'supporting color blending' came up on the forum: https://en.sfml-dev.org/forums/index.php?topic=24653

There is apparently one 'blend color' set by glBlendColor. It then gets used by some blending factors. Relevant ones are GL_CONSTANT_COLOR and probably/maybe also GL_ONE_MINUS_CONSTANT_COLOR, GL_CONSTANT_ALPHA and GL_ONE_MINUS_CONSTANT_ALPHA.

It should be easy enough to implement. Add an sf::Color field to sf::BlendMode, add 1-4 relevant enums, add/change caching and applying the sf::BlendMode in sf::RenderTarget.