Clear depth buffer, but only depth buffer for gears.

gears.c

@@ -174,6 +174,8 @@ gears_draw(struct gears *gears, GLfloat angle)
 {
 	GLfloat view_rotx = 20.0, view_roty = 30.0, view_rotz = 0.0;
 
+	glClear(GL_DEPTH_BUFFER_BIT);
+
 	glPushMatrix();
 
 	glTranslatef(0.0, 0.0, -50);

window.c

@@ -154,6 +154,7 @@ draw_window(struct window *window)
 						     window->config, window->name,
 						     window->width, window->height,
 						     window->stride, NULL);
+
 	if (surface == NULL)
 		die("failed to create surface\n");