Put back the glPopMatrix. This test was derived from blendminmax.c.

When extra code was cut from blendminmax.c's Display function, the
glPopMatrix call was also accidentally cut.

progs/tests/bug_3101.c

@@ -63,6 +63,8 @@ static void Display( void )
    glVertex2f(-1,  1);
    glEnd();
 
+   glPopMatrix();
+
    glutSwapBuffers();
 }