[media] vivid-tpg: precalculate downsampled lines

When dealing with vertical downsampling two successive lines have to be
averaged. In the case of the test pattern generator that only happens
if the two lines are using different patterns. So precalculate the average
between two pattern lines: one of pattern P and one of pattern P + 1.

That way there is no need to do any on-the-fly downsampling: it's all done
in the precalculate phase.

This patch also implements horizontal downsampling in the precalculate phase.
The only thing that needs to be done is to half the width since the actual
downsampling happens when two pixels at a time are generated.

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