리눅스 opengl 에서 glDrawBuffer(GL_FRONT) 로 그리면 Y값이 반전되는 현상

#include<stdio.h>
#include<stdlib.h>
#include<X11/X.h>
#include<X11/Xlib.h>
#include<GL/gl.h>
#include<GL/glx.h>
#include<GL/glu.h>
 
Display                 *dpy;
Window                  root;
GLint                   att[] = { GLX_RGBA, GLX_DEPTH_SIZE, 24, GLX_DOUBLEBUFFER, None };
XVisualInfo             *vi;
Colormap                cmap;
XSetWindowAttributes    swa;
Window                  win;
GLXContext              glc;
XWindowAttributes       gwa;
XEvent                  xev;
 
void DrawAQuad() {
 glClearColor(1.0, 1.0, 1.0, 1.0);
 glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
 glColor3f(1., 1., 0.);
 glBegin(GL_QUADS);		
	glVertex2f(  100.0f,   0.0f);
	glVertex2f(  100.0f,   100.0f);
	glVertex2f(  0.0f,   100.0f);
	glVertex2f(  0.0f,   0.0f);
glEnd();
} 
 
int main(int argc, char *argv[]) {
 
 dpy = XOpenDisplay(NULL);
 
 if(dpy == NULL) {
 	printf("\n\tcannot connect to X server\n\n");
        exit(0);
 }
 
 root = DefaultRootWindow(dpy);
 
 vi = glXChooseVisual(dpy, 0, att);
 
 if(vi == NULL) {
	printf("\n\tno appropriate visual found\n\n");
        exit(0);
 } 
 else {
	printf("\n\tvisual %p selected\n", (void *)vi->visualid); /* %p creates hexadecimal output like in glxinfo */
 }
 
 
 cmap = XCreateColormap(dpy, root, vi->visual, AllocNone);
 
 swa.colormap = cmap;
 swa.event_mask = ExposureMask | KeyPressMask;
 
 win = XCreateWindow(dpy, root, 0, 0, 600, 600, 0, vi->depth, InputOutput, vi->visual, CWColormap | CWEventMask, &swa);
 
 XMapWindow(dpy, win);
 XStoreName(dpy, win, "VERY SIMPLE APPLICATION");
 
 glc = glXCreateContext(dpy, vi, NULL, GL_TRUE);
 glXMakeCurrent(dpy, win, glc);
 
 glEnable(GL_DEPTH_TEST); 
 
 while(1) {
 	XNextEvent(dpy, &xev);
 
        if(xev.type == Expose) {
        	XGetWindowAttributes(dpy, win, &gwa);
            glViewport(0, 0, gwa.width, gwa.height);
			gluOrtho2D(-0.5, gwa.width-0.5, -0.5, gwa.height-0.5);
 
#if 0 // GL_BACK
			glDrawBuffer(GL_BACK);
        	DrawAQuad();
			glXSwapBuffers(dpy, win);
 
#else // GL_FRONT
			glDrawBuffer(GL_FRONT);
        	DrawAQuad(); 
			glFlush();
#endif
        }
 
	else if(xev.type == KeyPress) {
        	glXMakeCurrent(dpy, None, NULL);
 		glXDestroyContext(dpy, glc);
 		XDestroyWindow(dpy, win);
 		XCloseDisplay(dpy);
 		exit(0);
        }
    } /* this closes while(1) { */
} /* this is the } which closes int main(int argc, char *argv[]) { */