motif: Avoid constantly recalculating tile size.

motif: Avoid constantly recalculating tile size.

Instead of calculating tile size every time any drawing is needed,
we can just store the value calculated during window layout.  This
involves some code reorganization to give the ResizeGameArea action
implementation direct access to the state structure.

Moreover, avoid dealing with width/height separately in the drawing
functions, as by design the game/goal areas (and therefore the tiles)
are always square.