use unique_ptr to hold the Layers vector

src/canvas.h

@@ -165,7 +165,7 @@ public:
     std::unique_ptr<Stroke> m_current_stroke;
     std::unique_ptr<Stroke> m_dual_stroke;
     bool m_show_tmp = false;
-    std::vector<Layer> m_layers;
+    std::vector<std::unique_ptr<Layer>> m_layers;
     std::vector<int> m_order;
     std::vector<glm::vec2> m_plane_shape[6]; // screen space projection of the plane
     glm::mat4 m_plane_unproject[6] = SIXPLETTE(glm::mat4(1));