Constant amethyst_renderer::REPLACE

pub const REPLACE: Blend = Blend{color:
          BlendChannel{equation: Equation::Add,
                       source: Factor::One,
                       destination: Factor::Zero,},
      alpha:
          BlendChannel{equation: Equation::Add,
                       source: Factor::One,
                       destination: Factor::Zero,},}

When combining two fragments, choose the source value