add MP4Encoder class and test timelapse exporting

src/mp4enc.h

@@ -0,0 +1,33 @@
+#pragma once
+#include "image.h"
+
+class MP4Encoder
+{
+    struct Frame
+    {
+        enum class kType : uint8_t { IFrame, PFrame, Unknown };
+        kType type;
+        std::vector<uint8_t> data;
+    };
+    struct Header
+    {
+        std::vector<uint8_t> SPS_data;
+        std::vector<uint8_t> PPS_data;
+        uint8_t avc_profile;
+        uint8_t avc_profile_compat;
+        uint8_t avc_level;
+    };
+    class ISVCEncoder* m_encoder = nullptr;
+    int m_width = 0;
+    int m_height = 0;
+    int m_bitrate = 1000 << 10;
+    float m_framerate = 0;
+    Header m_header;
+    std::vector<Frame> m_frames;
+    std::vector<uint8_t> m_yuv_buffer;
+public:
+    bool init(int width, int height, int fps, int bitrate) noexcept;
+    bool encode(const Image& rgba) noexcept;
+    bool write_mp4(const std::string& filename) const noexcept;
+    void destroy() noexcept;
+};