Fix garbage pixels on the right of the board

Fix garbage pixels on the right of the board

The extra pixel that is requested in sizing the board window back to
an available size because we ofte get one less pixel than we ask for
can lead to a line of garbage pixels next to the board. This is combatted
by limiting expose events to the true board area, remembering that in the
Graph Option rather than the actual window size, and suppressing resizing
when the latter only exceeds the stored width by more than 1 pixel.