[media] vivid: add the kthread code that controls the video rate

Add the kthread handlers for video/vbi capture and video/vbi output.
These carefully control the rate at which frames are generated (video
capture) and accepted (video output). While the short-term jitter is
around the order of a jiffie, in the long term the rate matches the
configured framerate exactly.

The capture thread handler also takes care of the video looping and
of capture and overlay support. This is probably the most complex part
of this driver due to the many combinations of crop, compose and scaling
on the input and output, and the blending that has to be done if
overlay support is enabled as well.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>