motif: simplify border_clear implementation.

motif: simplify border_clear implementation.

This was cute, but since we no longer draw empty tiles in response to
expose events there are only two situations where the optimized border
clear is actually used:

  - when the game is won, or
  - when the window is resized.

In both cases the entire border needs to be cleared, meaning all this
code to reduce the number of drawing calls is optimizing for a case
that literally never happens.