renderer: re-use the same opengl instance for all background rendering

Previously the OpenGL instance was tied to each render/cache task,
creating and destroying it each time one started and stopped. This was
completely unnecessary since the instance holds no state and can be
shared by all of the render tasks without having to expensively start
a new one.

app/core.cpp

@@ -181,6 +181,9 @@ void Core::Start()
   //               the fact that some of the config paths set by default rely on the app name having been set (in main())
   Config::Current().SetDefaults();
 
+  // Load application config
+  Config::Load();
+
   // Declare custom types for Qt signal/slot system
   DeclareTypesForQt();
 
@@ -193,9 +196,8 @@ void Core::Start()
   // Initialize task manager
   TaskManager::CreateInstance();
 
-  // Load application config
-  Config::Load();
-
+  // Initialize OpenGL service
+  OpenGLProxy::CreateInstance();
 
   //
   // Start application
@@ -223,6 +225,8 @@ void Core::Stop()
     }
   }
 
+  OpenGLProxy::DestroyInstance();
+
   MenuShared::DestroyInstance();
 
   TaskManager::DestroyInstance();