Improve board drawing

Improve board drawing

The erasing of target-square markers could lead to flashing of the
moved piece. It was also ugly when the piece was slipped under a marker.
The actual display of the marker change on erasing can now be deferred
to a later position draw, and this is used when erasing after a drag-drop
move to do it after the move was accepted (so the piece would not be
flashed on the from-square, but still displayed without marker on it
after the drop). For click-click the erasure is shown before animation
starts. A problem with showing deselection when target-square marking
was off is now also fixed.