read-front: Be even meaner to DRI2 and choose read-buffer before swapping.

My first fix to intel fixed the previous iteration of read-front, but
glean was now complaining that it was reading the wrong results.  It looks
like DRI2 isn't catching the updates to the front due to swap in its fake
front.

tests/general/read-front.c

@@ -61,9 +61,10 @@ static void display()
 	glColor4fv(green);
 	piglit_draw_rect(win_width / 2, 0, win_width / 2, win_height);
 
-	glutSwapBuffers();
 	glReadBuffer(GL_FRONT);
 
+	glutSwapBuffers();
+
 	for (y = 0; y < win_height; y++) {
 		for (x = 0; x < win_width; x++) {
 			float *expected;