This commit fixes a regression that was most likely introduced in #678
which changed a bunch of stuff related to window and border dimension
calculation.
While we could previously assume that the points resize.right and
resize.bottom would always be 0 if we were dealing with a move rather
than a resize, these two points now depend on the values of BORDER_WIDTH
and BORDER_OFFSET. The code has been updated to reflect this and
calculate this constant just-in-time.