[media] vivid-tpg: add hor/vert downsampling support to tpg_gen_text

This will just skip lines/pixels since color fidelity is not quite
as important here as it is with the test patterns themselves.

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