vnc: Fix artifacts in hextile decoding

02c2b87 introduced a regression whereas the foreground color in a hextile
update was not being properly invalidated leading to artifacts.

It's still necessary to explicitly invalidate the foreground color with a
SubrectColoured tile even though we no longer send a foreground color as
part of the tile.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@siriusit.co.uk>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
