twn_draw.c: clear after swap

src/rendering/twn_draw.c

@@ -272,11 +272,11 @@ void render(void) {
         }
     }
 
-    clear_draw_buffer();
     render_space();
     render_skybox(); /* after space, as to use depth buffer for early rejection */
     render_2d();
     swap_buffers();
+    clear_draw_buffer();
 }