[media] vivid: add support for reduced frame rate in video capture

With this patch, vivid capture thread can now generate a video with
frame rate reduced by a factor of 1000 / 1001. This option can be
selected using a control Reduced Framerate from gui.

Signed-off-by: Prashant Laddha <prladdha@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>