title: Three.js Why renderer with viewport & scissor and I have a complete black canvas
body: <p>I have a typical UI - two vertical windows.
  The left for the canvas &amp; renderer, the right for the UI.</p>
  
  <p>Inside this UI I will have special viewers, for example a simple top view window.</p>
  
  <p>I have used the viewports &amp;&amp; scissors approach. All works fine.</p>
  
  <p>The question is that I miss some special parameter to avoid the complete canvas black-filled that covers all I have (my UI) behind. 
  I expected that viewport &amp; scissors were going to let me have 'clean' areas (not filled) inside my canvas. But not....</p>
  
  <p>The alpha approach ( <code>renderer = new THREE.WebGLRenderer( { alpha: true } );</code>) could be the fix but I don't know if this is the best solution (what about performance) or is there any way to program (enhancement) something related with the viewport &amp; scissors.</p>
  
  <p>If I'm going to use viewports there is no sense to fill all with black.</p>
  
  <p>(Also the main question is ... If I have a viewport &amp; scissor, why and who fill the canvas with black ?)</p>