libcamera creates memfds in two locations already, duplicating some code. Move the code to a new MemFd helper class. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Milan Zamazal <mzamazal@redhat.com> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com> Tested-by: Milan Zamazal <mzamazal@redhat.com> Tested-by: Hans de Goede <hdegoede@redhat.com> Reviewed-by: Hans de Goede <hdegoede@redhat.com>