libcamera: v4l2_device: Implement stream{On,Off}

Support starting and stopping a stream on a V4L2 device. Buffers must be
queued before the stream is started.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/include/v4l2_device.h

@@ -103,6 +103,9 @@ public:
 	int queueBuffer(Buffer *buffer);
 	Signal<Buffer *> bufferReady;
 
+	int streamOn();
+	int streamOff();
+
 private:
 	int getFormatSingleplane(V4L2DeviceFormat *format);
 	int setFormatSingleplane(V4L2DeviceFormat *format);