Constant draw_state::preset::blend::MULTIPLY

pub const MULTIPLY: Blend = Blend{color:
          BlendChannel{equation: Equation::Add,
                       source: Factor::ZeroPlus(BlendValue::DestColor),
                       destination: Factor::Zero,},
      alpha:
          BlendChannel{equation: Equation::Add,
                       source: Factor::ZeroPlus(BlendValue::DestAlpha),
                       destination: Factor::Zero,},}

When combining two fragments, multiply their values together.