Deep redesign of frame grabbers to prevent multiple frame captures

(slow) and optimize distribution of gabbed frames to multiple recording
and streaming pipelines (e.g. record H264 *and* share stream on the
network). New implementation of Loopback source for LINUX using
v4l2loopback (experimental).

Loopback.h

@@ -0,0 +1,31 @@
+#ifndef LOOPBACK_H
+#define LOOPBACK_H
+
+#include <vector>
+
+#include <gst/pbutils/pbutils.h>
+#include <gst/app/gstappsrc.h>
+
+#include "FrameGrabber.h"
+
+
+class Loopback : public FrameGrabber
+{
+    static std::string  system_loopback_pipeline;
+    static std::string  system_loopback_name;
+    static bool system_loopback_initialized;
+
+    void init(GstCaps *caps) override;
+    void terminate() override;
+
+public:
+
+    Loopback();
+
+    static bool systemLoopbackInitialized();
+    static bool initializeSystemLoopback();
+
+};
+
+
+#endif // LOOPBACK_H