add plugin camera desktop

plugins/camera_desktop/windows/camera_texture.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <flutter/texture_registrar.h>
+
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <vector>
+
+// Triple-buffer software texture for Windows.
+//
+// The capture thread calls Update() with new BGRA32 pixels.
+// The Flutter render thread calls the pixel-buffer callback to read frames.
+// Triple buffering avoids any locking between writer and reader:
+//   - write_idx  : capture thread writes here
+//   - ready_idx  : swapped by capture thread after write (latest frame)
+//   - read_idx   : Flutter render thread reads from here
+class CameraTexture {
+ public:
+  explicit CameraTexture(flutter::TextureRegistrar* registrar);
+  ~CameraTexture();
+
+  // Registers the texture with Flutter and returns the texture ID.
+  int64_t Register();
+
+  // Updates the texture with a new BGRA32 frame.
+  // Called from the capture thread.
+  void Update(const uint8_t* bgra, int width, int height);
+
+  // Unregisters the texture from Flutter.
+  void Unregister();
+
+  // Asynchronously unregisters the texture; |on_done| runs once Flutter has removed
+  // it (on the raster thread). The caller MUST keep this object alive until then,
+  // because Flutter's pixel-buffer callback can still fire on the raster thread after
+  // an ordinary Unregister() returns. See CRASH.md.
+  void UnregisterAsync(std::function<void()> on_done);
+
+  int64_t texture_id() const { return texture_id_; }
+
+ private:
+  const FlutterDesktopPixelBuffer* ObtainPixelBuffer(size_t width,
+                                                     size_t height);
+
+  flutter::TextureRegistrar* registrar_;
+  std::unique_ptr<flutter::TextureVariant> texture_variant_;
+  int64_t texture_id_ = -1;
+
+  // Triple buffer, same pattern as linux/camera_texture.cc.
+  std::vector<uint8_t> bufs_[3];
+  int write_idx_ = 0;
+  int ready_idx_ = 1;
+  int read_idx_ = 2;
+  bool has_new_frame_ = false;
+  std::mutex mutex_;
+
+  int width_ = 0;
+  int height_ = 0;
+
+  FlutterDesktopPixelBuffer pixel_buffer_{};
+};