Does anyone know how to resolve this fullscreen problem?

Bad: http://img33.picoodle.com/data/img33...tm_7bd8591.png

Good: http://img29.picoodle.com/data/img29...tm_62b6140.png

I'm using SDL, the problem only occurrs when a non-native resolution is used as fullscreen resolution.

Code :
void RenderResize() {
    glViewport(0,0,r_width,r_height);
	GLfloat ratio=GLfloat(r_width)/GLfloat(r_height==0?1:r_height);
 
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(45.0f,ratio,0.1f,100.0f);
	glMatrixMode(GL_MODELVIEW);
}
 
void RenderInit() {
	glEnable(GL_TEXTURE_2D);
 
	glShadeModel(GL_SMOOTH);
 
	glClearColor(0.4f,0.4f,0.4f,1.0f);
 
	glClearDepth(1.0f);
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_ALPHA_TEST);
	glAlphaFunc(GL_GREATER,0.5);
	glDepthFunc(GL_LEQUAL);
}
 
void RenderUpdate() {
	glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
 
    glRotatef(GLfloat(CamPitch),1.0f,0.0f,0.0f);
    glRotatef(GLfloat(CamYaw),0.0f,1.0f,0.0f);
	gluLookAt(CamX,CamY,CamZ,CamX,CamY,CamZ-1,0.0,1.0,0.0);
 
	Shield.Relocate();
	m_shield.render();
	m_shield.render_box();
	Shield.Return();
 
	Goat.Relocate();
	m_goat.render();
	m_goat.render_box();
	Goat.Return();
 
	SDL_GL_SwapBuffers();
}