Constant draw_state::preset::blend::ADD

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

When combining two fragments, add their values together, saturating at 1.0