refactor to allow various gfx backends (work-in-progress)

btgui/OpenGLWindow/SimpleOpenGL3App.cpp

@@ -140,6 +140,7 @@ SimpleOpenGL3App::SimpleOpenGL3App(	const char* title, int width,int height)
     b3Assert(glGetError() ==GL_NO_ERROR);
 
 	m_instancingRenderer = new GLInstancingRenderer(128*1024,32*1024*1024);
+	m_renderer = m_instancingRenderer ;
 	m_instancingRenderer->init();
 	m_instancingRenderer->resize(width,height);