libcamera: v4l2_videodevice: Better tracking of the device state

Replace the existing streaming_ state variable with an enum to track the
following three state: Streaming, Stopping, and Stopped. The alternate states
will be used in a subsequent commit.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/v4l2_videodevice.h

@@ -225,6 +225,12 @@ protected:
 private:
 	LIBCAMERA_DISABLE_COPY(V4L2VideoDevice)
 
+	enum class State {
+		Streaming,
+		Stopping,
+		Stopped,
+	};
+
 	int getFormatMeta(V4L2DeviceFormat *format);
 	int trySetFormatMeta(V4L2DeviceFormat *format, bool set);
 
@@ -258,7 +264,7 @@ private:
 
 	EventNotifier *fdBufferNotifier_;
 
-	bool streaming_;
+	State state_;
 };
 
 class V4L2M2MDevice