add plugin camera desktop

plugins/camera_desktop/linux/photo_handler.h

@@ -0,0 +1,18 @@
+#ifndef PHOTO_HANDLER_H_
+#define PHOTO_HANDLER_H_
+
+#include <gst/gst.h>
+#include <string>
+
+class PhotoHandler {
+ public:
+  // Captures a still image from the appsink's last-sample property (read-only,
+  // no consumer conflict with the preview stream). Converts the RGBA frame to
+  // JPEG via gst_video_convert_sample and writes it to |output_path|.
+  // Returns true on success; sets |error| on failure.
+  static bool TakePicture(GstElement* appsink,
+                          const std::string& output_path,
+                          GError** error);
+};
+
+#endif  // PHOTO_HANDLER_H_