message: Fix environment preview for viewer
  The environment material used for 3d preview was not being recompiled when changing the environment.
  Hence the scene was rendering using the new environment, but the preview was not.
diff: @@ -479,6 +479,9 @@ void Viewer::loadEnvironmentLight()
  _lightRigDoc = nullptr;
  }
  }
  +
  + // Invalidate the existing environment material, if any.
  + _envMaterial = nullptr;
  }
  void Viewer::applyDirectLights(mx::DocumentPtr doc)
  