UI: Fix Xv painter rendering of OSD menu gradient elements

Commit #xxxxx re-worked and simplified MythPainter.
MythYUVAPainter::DrawRect is passed a brush to paint the background.
This brush may be configured to gradient fill - for instance in
the MythCentre theme to draw the MythPopupBox OSD menu buttons.
Currently only the basic brush colour is translated to YUVA so the
OSD menu is incorrectly rendered.

This patch translates the colours in the brush fill gradient from
RGB to YUV.

Signed-off-by: Lawrence Rust <lvr@softsystem.co.uk>
Signed-off-by: Stuart Morgan <smorgan@mythtv.org>
Fix --update behaviour in mythfilldatabase after b7a1bd237

This also deprecates --update in favour of the clearer
--only-update-guide which does the same job but is more descriptive
and mirrors --only-update-channels