Improve exposure mask calculation.

Improve exposure mask calculation.

By assuming that a >= b, the computation of

  board_left(a) & board_right(b)

can be simplified to

  board_left(a-b) << a.

which avoids some computation.  A similar improvement can be done
for board_above/board_below.  Add a new board_rect function that
incorporates these improvements, and use it to generate exposure
update bitmaps in the Motif UI.

A new test case exhaustively tests all possible inputs to this
function.

Furthermore, update board_above and board_below to have simpler
implementations.