Some sensors (namely the Sony IMX296, whose support will be added shortly) require different conversion formulas between exposure time and exposure lines. Make the Exposure() and ExposureLines() functions virtual to allow this. Signed-off-by: Naushir Patuck <naush@raspberrypi.com> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Naushir Patuck <naush@raspberrypi.com> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>