fix mixing up of SDL and libc allocators, proper flushing of quad build buffers

src/rendering/twn_rects.c

@@ -90,6 +90,8 @@ void render_rect_batch(const Primitive2D primitives[],
                 (Vec2){0}, (Vec2){0}, (Vec2){0}, (Vec2){0},
                 rect.color);
         }
+
+        finish_vertex_builder(&payload);
     }
 
     finally_render_quads(primitives, batch, vertex_array);