free resources on app destruction

src/canvas.h

@@ -13,6 +13,8 @@
 class Layer
 {
 public:
+    //Layer() = default;
+    //Layer(const Layer&) = delete;
     RTT m_rtt[6];
     glm::vec4 m_dirty_box[6] = SIXPLETTE(glm::vec4(0));
     bool m_dirty_face[6] = SIXPLETTE(false);
@@ -223,6 +225,8 @@ public:
     std::vector<Layer::Snapshot> m_layers_snapshot;
 
     Canvas() { I = this; }
+    ~Canvas() { destroy(); }
+    void destroy();
     bool create(int width, int height);
     void resize(int width, int height);
     void layer_remove(int idx);