title: Issue with adding shadow to a SCNPlane with clear background
body: <p>I am trying to add a shadow on <code>SCNPlane</code>, everything works fine but I cannot make SCNPlane transparent to show only the shadow not with the white background. here is the code:</p>
  
  <pre><code>        let flourPlane = SCNPlane()
          let groundPlane = SCNNode()
  
  
          let clearMaterial = SCNMaterial()
          clearMaterial.lightingModel = .constant
          //clearMaterial.colorBufferWriteMask = []
          clearMaterial.writesToDepthBuffer = true
          clearMaterial.transparencyMode = .default
  
          flourPlane.materials = [clearMaterial]
  
          groundPlane.scale = SCNVector3(200, 200, 200)
          groundPlane.geometry = flourPlane
          groundPlane.castsShadow = false
          groundPlane.eulerAngles = SCNVector3Make(-Float.pi/2, 0, 0)
          groundPlane.position = SCNVector3(x: 0.0, y: shadowY, z: 0.0)
          node.addChildNode(groundPlane)
  
          // Create a ambient light
          let ambientLight = SCNNode()
          ambientLight.light = SCNLight()
          ambientLight.light?.shadowMode = .deferred
          ambientLight.light?.color = UIColor.white
          ambientLight.light?.type = SCNLight.LightType.ambient
          ambientLight.position = SCNVector3(x: 0,y: 5,z: 0)
  
          // Create a directional light node with shadow
           let myNode = SCNNode()
          myNode.light = SCNLight()
          myNode.light?.type = .directional
          myNode.light?.castsShadow = true
          myNode.light?.automaticallyAdjustsShadowProjection = true
          myNode.light?.shadowSampleCount = 80
          myNode.light?.shadowBias = 1
          myNode.light?.orthographicScale = 1
          myNode.light?.shadowMode = .deferred
          myNode.light?.shadowMapSize = CGSize(width: 2048, height: 2048)
          myNode.light?.shadowColor = UIColor.black.withAlphaComponent(0.5)
          myNode.light?.shadowRadius = 10.0
          myNode.eulerAngles = SCNVector3Make(-Float.pi/2, 0, 0)
          node.addChildNode(ambientLight)
          node.addChildNode(myNode)
  </code></pre>
  
  <p>When I add <code>clearMaterial.colorBufferWriteMask = []</code> shadow disappears! how can create a transparent material to show only the shadow.</p>
  
  <p><a href="https://i.stack.imgur.com/jy0fg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jy0fg.png" alt="enter image description here"></a></p>
  
  <p>The white area is SCNPlane and the red is the background.</p>