title: Custom SCNGeometry not displaying diffuse contents as texture
body: <p>I'm creating a custom SCNGeometry. To start, I'm developing it as a flat plane. It displays, and I can apply a colour to its' diffuse contents, but if I try and apply anything such as a UIImage, or a CALayer, it displays as white.</p>
  
  <p>Code:</p>
  
  <pre><code>let positions = [
      SCNVector3(-1, 0, 0),
      SCNVector3(1, 0, 0),
      SCNVector3(-1, 0, -1),
      SCNVector3(1, 0, -1)
  ]
  
  let indices: [UInt8] = [
      0, 1, 2,
      1, 3, 2
  ]
  
  let vertexSource = SCNGeometrySource(vertices: positions)
  
  let indexData = Data(bytes: indices, count: indices.count)
  
  let element = SCNGeometryElement(
      data: indexData,
      primitiveType: SCNGeometryPrimitiveType.triangles,
      primitiveCount: indices.count / 3,
      bytesPerIndex: 1)
  
  let geometry = SCNGeometry(
      sources: [vertexSource],
      elements: [element])
  
  geometry.firstMaterial?.diffuse.contents = UIImage(named: "homer")
  </code></pre>