miext/damage: take care of the coordinate mode in damagePolyPoint

The mode (CoordModeOrigin or CoordModePrevious) was not taken into
account when computing the box. The result was a bad drawing of
points in some situations (on my hardware/software configuration,
calling XDrawString followed by XDrawPoints in the mode
CoordModePrevious).

Signed-off-by: Cedric Roux <sed@free.fr>
Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit bc36594e)