libcamera: v4l2_videodevice: Identify non-zero stream starts

V4L2 Video devices should always start streaming from index zero.
Identify and report a warning if they don't, and correct for any offset.

Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/v4l2_videodevice.h

@@ -276,6 +276,7 @@ private:
 	EventNotifier *fdBufferNotifier_;
 
 	State state_;
+	std::optional<unsigned int> firstFrame_;
 
 	Timer watchdog_;
 	utils::Duration watchdogDuration_;