title: draw not continuous line on drag mouse with OpenGL
body: <p>I want to make simple painter program so I studying OpenGL.
  Today I practiced draw line with <code>GL_POINTS</code> and <code>GL_LINE_STRIP</code>. and here is some problem.
  for example </p>
  
  <p><img src="https://i.imgur.com/cUe54ox.png"></p>
  
  <p>I want to draw this way. but If I use <code>glBegin(GL_LINE_STRIP)</code></p>
  
  <p>It looks like this image.</p>
  
  <p><img src="https://i.imgur.com/AJ6WlcO.png"></p>
  
  <p>every line is continuous.
  How to remove that lines? Is it difficult?
  (anyway sorry for my English... )</p>