add export timelapse

src/mp4enc.h

@@ -34,6 +34,7 @@ public:
     bool write_mp4(const std::string& filename) const noexcept;
     void destroy() noexcept;
     int frames_count() const noexcept { return m_frames.size(); }
+    glm::ivec2 frame_size() const noexcept { return { m_width, m_height }; }
     virtual bool read(BinaryStreamReader& r) override;
     virtual void write(BinaryStreamWriter& w) const override;
 };