use PBO for timelapse data read

src/rtt.h

@@ -1,6 +1,36 @@
 #pragma once
 #include "image.h"
 
+struct PBO
+{
+    // copy
+    PBO(const PBO&) = delete;
+    PBO& operator=(const PBO&) = delete;
+    // move
+    PBO(PBO&& other) noexcept;
+    PBO& operator=(PBO&&) noexcept;
+    // default
+    PBO() noexcept = default;
+    ~PBO() noexcept;
+
+    GLuint buffer_id = 0;
+    GLenum bound_slot = 0;
+    GLubyte* mapped_ptr = nullptr;
+
+    int width = 0;
+    int height = 0;
+
+    bool create() noexcept;
+    bool create(class RTT& rtt) noexcept;
+    void destroy() noexcept;
+/*
+    void bind_read() noexcept;
+    void unbind() noexcept;
+*/
+    uint8_t* map() noexcept;
+    void unmap() noexcept;
+};
+
 class RTT
 {
     int w = 0;