Implement customFocusPoint in camerabin backend.

Send a regions-of-interest event containing a rectangle surrounding the
focus point to the camera source when the camera's focusPointMode
is QCameraFocus::CustomFocusPoint.  Reset the regions-of-interest if the
focusPointMode changes to any other value.

[ChangeLog][GStreamer] Implement customFocusPoint in camerabin backend.

Change-Id: If75405733b18d476dff5467abae3f08271caf06a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/plugins/gstreamer/camerabin/camerabinsession.h

@@ -103,6 +103,7 @@ public:
     GstPhotography *photography();
 #endif
     GstElement *cameraBin() { return m_camerabin; }
+    GstElement *cameraSource() { return m_videoSrc; }
     QGstreamerBusHelper *bus() { return m_busHelper; }
 
     CameraRole cameraRole() const;