Dynamic loading of the Dispmanx Grabber (#1440)

* BCM dynamic loading

* cleanup

* better handle start of dispmanx

* set GrabberWrapper pointer to null on exit

* Update DispmanxFrameGrabber.cpp

include/grabber/DispmanxWrapper.h

@@ -25,6 +25,13 @@ public:
 
 	bool screenInit();
 
+	bool available = false;
+
+	///
+	/// Starts the grabber which produces led values with the specified update rate
+	///
+	bool open() override;
+
 public slots:
 	///
 	/// Performs a single frame grab and computes the led-colors