Fix loading render engine plugins in GUI (#1694)

Signed-off-by: Ian Chen <ichen@osrfoundation.org>

include/ignition/gazebo/rendering/RenderUtil.hh

@@ -201,6 +201,10 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     /// \brief Clears the set of selected entities and lowlights all of them.
     public: void DeselectAllEntities();
 
+    /// \brief Init render engine plugins paths. This lets gz-rendering know
+    /// paths to find render engine plugins
+    public: void InitRenderEnginePluginPaths();
+
     /// \brief Helper function to get all child links of a model entity.
     /// \param[in] _entity Entity to find child links
     /// \return Vector of child links found for the parent entity

src/gui/plugins/scene_manager/GzSceneManager.cc

@@ -59,6 +59,9 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     /// \brief Rendering utility
     public: RenderUtil renderUtil;
 
+    /// \brief True if render engine plugins paths are initialized
+    public: bool renderEnginePluginPathsInit{false};
+
     /// \brief List of new entities from a gui event
     public: std::set<Entity> newEntities;
 
@@ -123,6 +126,12 @@ void GzSceneManager::Update(const UpdateInfo &_info,
 
   IGN_PROFILE("GzSceneManager::Update");
 
+  if (!this->dataPtr->renderEnginePluginPathsInit)
+  {
+    this->dataPtr->renderUtil.InitRenderEnginePluginPaths();
+    this->dataPtr->renderEnginePluginPathsInit = true;
+  }
+
   this->dataPtr->renderUtil.UpdateECM(_info, _ecm);
 
   std::lock_guard<std::mutex> lock(this->dataPtr->newRemovedEntityMutex);

src/rendering/RenderUtil.cc

@@ -2496,16 +2496,22 @@ bool RenderUtil::HeadlessRendering() const
   return this->dataPtr->isHeadlessRendering;
 }
 
+/////////////////////////////////////////////////
+void RenderUtil::InitRenderEnginePluginPaths()
+{
+  ignition::common::SystemPaths pluginPath;
+  pluginPath.SetPluginPathEnv(kRenderPluginPathEnv);
+  rendering::setPluginPaths(pluginPath.PluginPaths());
+}
+
 /////////////////////////////////////////////////
 void RenderUtil::Init()
 {
   // Already initialized
   if (nullptr != this->dataPtr->scene)
     return;
 
-  ignition::common::SystemPaths pluginPath;
-  pluginPath.SetPluginPathEnv(kRenderPluginPathEnv);
-  rendering::setPluginPaths(pluginPath.PluginPaths());
+  this->InitRenderEnginePluginPaths();
 
   std::map<std::string, std::string> params;
   if (this->dataPtr->useCurrentGLContext)