message: Viewer shadow map fix
  This changelist addresses an issue with shadow mapping for scenes with multiple meshes in the MaterialX viewer.  Previously, only the first scene mesh was used in computing shadow bounds, and now all scene meshes are taken into account.
diff: @@ -2295,10 +2295,9 @@ void Viewer::updateCameras()
  mx::NodePtr dirLight = _lightHandler->getFirstLightOfCategory(DIR_LIGHT_NODE_CATEGORY);
  if (dirLight)
  {
  - mx::MeshPtr mesh = _geometryHandler->getMeshes()[0];
  - float r = mesh->getSphereRadius();
  - _shadowCamera->setWorldMatrix(meshRotation *
  - mx::Matrix44::createTranslation(-mesh->getSphereCenter()));
  + mx::Vector3 sphereCenter = (_geometryHandler->getMaximumBounds() + _geometryHandler->getMinimumBounds()) * 0.5;
  + float r = (sphereCenter - _geometryHandler->getMinimumBounds()).getMagnitude();
  + _shadowCamera->setWorldMatrix(meshRotation * mx::Matrix44::createTranslation(-sphereCenter));
  _shadowCamera->setProjectionMatrix(mx::Camera::createOrthographicMatrix(-r, r, -r, r, 0.0f, r * 2.0f));
  mx::ValuePtr value = dirLight->getInputValue("direction");
  if (value->isA<mx::Vector3>())
  