partial implementation of double buffered render

src/rendering/twn_draw.c

@@ -374,11 +374,11 @@ void render(void) {
         }
     }
 
-    render_space();
-    render_skybox(); /* after space, as to use depth buffer for early rejection */
-    render_2d();
-    swap_buffers();
-    clear_draw_buffer();
+    start_render_frame(); {
+        render_space();
+        render_skybox(); /* after space, as to use depth buffer for early z rejection */
+        render_2d();
+    } end_render_frame();
 }