while(1) {

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(...);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(...);

適当に物体描写
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(...);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(...);

適当にHUD描写

glFlush();
}