glClearColor in vtkOpenGLGPUVolumeRayCastMapper::vtkInternal::SetupRenderToTexture()
In fuction vtkOpenGLGPUVolumeRayCastMapper::vtkInternal::SetupRenderToTexture(), use glClear's value is not normal. In orther case is glClearColor(0, 0, 0, 0), but in this, we use glClearColor(1, 1, 1, 0), which will lead to white borders in rendered image.