partial implementation of double buffered render

src/rendering/twn_rects.c

@@ -72,9 +72,7 @@ void render_rect_batch(const Primitive2D primitives[],
     SDL_assert(primitives[0].type == PRIMITIVE_2D_RECT);
 
     /* single vertex array is used for every batch with NULL glBufferData() trick at the end */
-    static VertexBuffer vertex_array = 0;
-    if (vertex_array == 0)
-        vertex_array = create_vertex_buffer();
+    VertexBuffer const vertex_array = get_scratch_vertex_array();
 
     use_texture_mode(batch.mode);