refine sample

the camera's color must be set as A: 0, V: 0 so that the background can become transparent

Samples/UIWidgetsSamples_2019_4/Assets/Resources/CubPrefab.prefab

   m_GameObject: {fileID: 606295237506465389}
   m_Enabled: 1
   serializedVersion: 2
-  m_ClearFlags: 1
-  m_BackGroundColor: {r: 0.19215687, g: 0.3019608, b: 0.4745098, a: 0}
+  m_ClearFlags: 2
+  m_BackGroundColor: {r: 0, g: 0, b: 0, a: 0}
   m_projectionMatrixMode: 1
   m_GateFitMode: 2
   m_FOVAxisMode: 0

Samples/UIWidgetsSamples_2019_4/Assets/Script/ModelViewHelper.cs

             
             var gameObj = (GameObject) Instantiate(modelViewPrefab, Vector3.zero, Quaternion.identity);
             
-            var renderTexture = new RenderTexture(100, 100, 24);
+            var renderTexture = new RenderTexture(100, 100, 32);
 
             gameObj.transform.Find("Camera").GetComponent<Camera>().targetTexture = renderTexture;
             

Samples/UIWidgetsSamples_2019_4/Assets/Script/Scene3DTest1.cs

                         children: new List<Widget>
                         {
                             new Lottie("wine.json", size: new Size(100, 100)),
-                            new Container(width: 100, height: 100, child:new Texture(texture: text)),
+                            new Container(width: 200, height: 200, color: Colors.red, child: new Center(child: new Container(width: 100, height: 100, child:new Texture(texture: text)))),
                             new Container(
                                 width: 100,
                                 height: 100,