26. More blending equations

• The program can change the underlying equation as well using glBlendEquation() parameter:

  
      GL_FUNC_ADD               Cf = ( Cs * S ) + ( Cd * D )
      GL_FUNC_SUBTRACT          Cf = ( Cs * S ) - ( Cd * D )
      GL_FUNC_REVERSE_SUBTRACT  Cf = ( Cd * D ) - ( Cs * S )
      GL_MIN                    Cf = min( Cs, Cd )
      GL_MAX                    Cf = max( Cs, Cd )