bool bSky = false;

// В glBegin:
if (mode == GL_QUADS)
	bSky = true;
else
	bSky = false;

// В glClear:
if ((mask==GL_DEPTH_BUFFER_BIT) && bSky)
{
	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
	mask = GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT;
	glClear(mask);
}

// В glVertex3fv
if (bSky == true)
	return;