Adding a clipping plane to vtkOpenGLGPUVolumeRaycastMapper in order to handle camera near plane clipping is currently broken in XR and users can see through the volume when placing the head inside it. * The [`vtkOpenGLGPUVolumeRaycastMapper::IsCameraInside`](https://gitlab.kitware.com/vtk/vtk/-/blob/master/Rendering/VolumeOpenGL2/vtkOpenGLGPUVolumeRayCastMapper.cxx#L994) check fails because the frustum planes returned by a VRCamera are wrong (physical vs world) * If not overridden in VRCamera, `GetFrustumPlanes` calls [`GetCompositeProjectionTransformMatrix`](https://gitlab.kitware.com/vtk/vtk/-/blob/master/Rendering/Core/vtkCamera.cxx#L1296) which ignores the stereo matrices and computes regular camera projection. * Overriding ComputeProjectionTransform as follow fixes the clipping issue, but computing a regular projection matrix is still required for hardware picking. TODO: Find a better approach that using "LastRenderer", or confirm LastRenderer is a good candidate to check the TrackHMD flag. ```  if (!this->LastRenderer || !this->LastRenderer->GetRenderWindow() || vtkVRRenderWindow::SafeDownCast(this->LastRenderer->GetRenderWindow())->GetTrackHMD())  {    this->ProjectionTransform->Identity();    if (this->LeftEye)     this->ProjectionTransform->Concatenate(this->LeftEyeToProjectionMatrix);    else     this->ProjectionTransform->Concatenate(this->RightEyeToProjectionMatrix);  }  else  {    this->Superclass::ComputeProjectionTransform(aspect, nearz, farz);  } ```