V4L/DVB (9235): Precalculate vivi yuv values

This patch improves the color space conversion code in vivi.c to
directly draw with precalculated YUV values as palette instead of
drawing with YUV that is calculated from RGB for every two pixels.
This way we eliminate the need for 9 multiplications every two pixels.

A side effect of this patch is that the time counter is changed from
green text on black background to white text on black background.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>