destroy all textures on window destruction and restore OpenGL objects when Android resumes the app

engine/canvas.h

@@ -16,6 +16,10 @@ public:
     bool m_locked = false;
     float m_opacity = 1.f;
     std::string m_name;
+    struct Snapshot
+    {
+        std::unique_ptr<uint8_t[]> image[6];
+    };
     bool create(int width, int height, std::string name)
     {
         for (int i = 0; i < 6; i++)
@@ -44,6 +48,16 @@ public:
         // restore clear color state
         glClearColor(cc[0], cc[1], cc[2], cc[3]);
     }
+    Snapshot snapshot()
+    {
+        Snapshot snap;
+        for (int i = 0; i < 6; i++)
+        {
+            snap.image[i] = std::make_unique<uint8_t[]>(m_rtt[i].bytes());
+            m_rtt[i].readTextureData(snap.image[i].get());
+        }
+        return snap;
+    }
 };
 
 class Canvas
@@ -78,6 +92,8 @@ public:
     Sampler m_sampler_bg;
     glm::vec2 m_cam_rot;
     float m_cam_fov = 85;
+    
+    std::vector<Layer::Snapshot> m_layers_snapshot;
 
     bool create(int width, int height);
     void resize(int width, int height);
@@ -89,6 +105,8 @@ public:
     void stroke_end();
     void stroke_commit();
     void clear(const glm::vec4& color = { 1, 1, 1, 1 });
+    void snapshot_save();
+    void snapshot_restore();
 };