Fix preview rendering (#48)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphPreviewGenerator.cs

 
             Unsupported.SetOverrideRenderSettings(m_Scene);
 
+            RenderTexture.active = m_RenderTexture;
+            GL.Clear(true, true, Color.black);
-
             if (mode == PreviewMode.Preview3D)
             {
                 m_Camera.transform.position = -Vector3.forward * 5;
                 m_Camera.orthographicSize = 1;
                 m_Camera.orthographic = true;
             }
+
+            m_Camera.targetTexture = m_RenderTexture;
 
             EditorUtility.SetCameraAnimateMaterialsTime(m_Camera, time);
             Light0.enabled = true;