vnc: Fix artifacts in hextile decoding

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 <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>