0024228: TKOpenGL - destroy GL context at view close

- OpenGl_Display - release GL resources correctly on closing views
- OpenGl_AspectFace, OpenGl_AspectText, OpenGl_AspectLine, OpenGl_AspectMarker - initialize OpenGl resources on demand, when context is available.
- Graphic3d_TextureRoot - use const modifier for GetId method to avoid asynchronous resource state at OpenGl.
- Do not call OpenGL functions if no active GL context has been left
- Reset thread's context before deletion for Mesa WNT

src/OpenGl/OpenGl_Context.hxx

@@ -120,6 +120,12 @@ public:
   //! GL context should be active!
   Standard_EXPORT Standard_Boolean Init();
 
+  //! @return true if this context is valid (has been initialized)
+  inline Standard_Boolean IsValid() const
+  {
+    return myIsInitialized;
+  }
+
 #if defined(_WIN32)
   Standard_EXPORT Standard_Boolean Init (const Aspect_Handle           theWindow,
                                          const Aspect_Handle           theWindowDC,