glClear() causing crash on alt+tab.

void GraphicsSystem::oglinit(){
	SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );			// tell SDL that the GL drawing is going to be double buffered
	SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE,   32);			// size of depth buffer
	SDL_GL_SetAttribute( SDL_GL_STENCIL_SIZE, 0);			// we aren't going to use the stencil buffer
	SDL_GL_SetAttribute( SDL_GL_ACCUM_RED_SIZE, 0);			// this and the next three lines set the bits allocated per pixel -
	SDL_GL_SetAttribute( SDL_GL_ACCUM_GREEN_SIZE, 0);		// - for the accumulation buffer to 0
	SDL_GL_SetAttribute( SDL_GL_ACCUM_BLUE_SIZE, 0);
	SDL_GL_SetAttribute( SDL_GL_ACCUM_ALPHA_SIZE, 0);
	glClearColor (0.0f, 0.0f, 0.0f, 0.5f);						// Black Background
	glClearDepth (1.0f);										// Depth Buffer Setup
	glDepthFunc  (GL_LEQUAL);									// The Type Of Depth Testing (Less Or Equal)
	glEnable     (GL_DEPTH_TEST);								// Enable Depth Testing
	glShadeModel (GL_SMOOTH);									// Select Smooth Shading
	glHint       (GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);	// Set Perspective Calculations To Most Accurate
	glEnable     (GL_TEXTURE_2D);								// Enable Texture Mapping
//	glEnable     (GL_CULL_FACE);								// Remove Back Face
	gr = ((1 + sqrt((double)5)) / 2);
}

void GraphicsSystem::Render(std::vector<int> scenegraph,Player* player){
	output = fopen("GLErrorLog.txt","w");
	fprintf(output,"%s\n",gluErrorString(glGetError()));	
	fclose(output);

	output = fopen("SDLErrorLog.txt","w");
	fprintf(output,"%s\n",SDL_GetError());	
	fclose(output);
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);		// clear screen and depth buffer
	glLoadIdentity();										// reset modelview matrix
	glRotatef(player->xangle, 1.0f, 0.0f, 0.0f);
	glRotatef(player->yangle, 0.0f, 1.0f, 0.0f);
	glRotatef(player->zangle, 0.0f, 0.0f, 1.0f);
	glTranslatef(player->x,player->y,player->z);

	glColor3f(0.0f,0.6f,0.0f);
	glBegin(GL_LINES);

	for (float i = -100; i < 100;i++){
		glVertex3f(i,0,100);
		glVertex3f(i,0,-100);
	}
	for (float i = -100; i < 100;i++){
		glVertex3f(100,0,i);
		glVertex3f(-100,0,i);
	}
	glEnd();

	glBegin(GL_POINTS);
		glColor3f(1,0,0);
		glVertex3f(0,1,gr);
		glVertex3f(0,1,-gr);
		glVertex3f(0,-1,gr);
		glColor3f(0,1,0);
		glVertex3f(0,-1,-gr);
		
		glVertex3f(1,gr,0);
		glVertex3f(1,-gr,0);
		glColor3f(0,0,1);
		glVertex3f(-1,gr,0);
		glVertex3f(-1,-gr,0);

		glVertex3f(gr,0,1);
		glColor3f(1,1,1);
		glVertex3f(gr,0,-1);
		glVertex3f(-gr,0,1);
		glVertex3f(-gr,0,-1);
	glEnd();
	SDL_GL_SwapBuffers();
}

void GraphicsSystem::oglinit3D(float fov,float aspect,float zNear,float zFar){
	glMatrixMode   (GL_PROJECTION);								// Select The Projection Matrix
	glLoadIdentity ();											// Reset The Projection Matrix
	gluPerspective(fov,aspect,zNear,zFar);
	glMatrixMode   (GL_MODELVIEW);								// Select The Modelview Matrix
//90,1,0.1f,100.0f
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);	// Nearest Filtered
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);	// Nearest Filtered
}

window::window(int width, int height,std::string WindowName){
	done = false;
	if ( SDL_Init( SDL_INIT_VIDEO ) < 0 )
	{
		fprintf( stderr, "Video initialization failed: %s\n",
			SDL_GetError( ) );
		SDL_Quit( );
	}
	atexit(SDL_Quit);
	SDL_WM_SetCaption(WindowName.c_str(), NULL);
	screen = SDL_SetVideoMode(width, height, 0, SDL_FULLSCREEN|SDL_OPENGL);
	if( !screen ) {
		fprintf(stderr, "Couldn't create a surface: %s\n",
			SDL_GetError());
		done = true;
	}
	SDL_ShowCursor(false);

	Player* player = new Player();						//Create new player
	GameTimer* timer = new GameTimer();					//Create new timer object
	GraphicsSystem* graphics = new GraphicsSystem();	//Create graphics object
	GameInput* input = new GameInput();					//Create game input object
	GameLogic* logic = new GameLogic();					//Create game logic object
	timer->Update();
	graphics->oglinit();
	graphics->oglinit3D(90,((float)width/(float)height),0.1f,100);

	std::vector<int> sg;

	while(!done)
	{	
		SDL_Event event;

		while ( SDL_PollEvent(&event) )
		{
			switch (event.type)
			{
			case SDL_QUIT:
				done = true;
				break;
			case SDL_KEYDOWN:
				if ( event.key.keysym.sym == SDLK_ESCAPE ) { done = 1; }
				break;
			}
		}
		input->Update();
		logic->UpdateGame(input->GetKeyState(),timer->GetDifference(),player,sg);
		graphics->oglinit3D(90,(float)(width/height),0.1f,100);
		graphics->Render(sg,player);
		timer->Update();
	}
	delete player;
	delete timer;
	delete graphics;
	delete input;
	delete logic;
}