OpenXR: Fix near plane clipping with camera inside volume

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 check fails because the frustum planes returned by a VRCamera are wrong (physical vs world)
• If not overridden in VRCamera, GetFrustumPlanes calls GetCompositeProjectionTransformMatrix 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);
  }