Make Box::is_empty more selective and create an axis-specific variant

Make Box::is_empty more selective and create an axis-specific variant

Boxes are not really "empty" just because they show negative
advancement on some axis.  A box is empty if its original dimensions
of (+inf . -inf) are still unchanged for both axes.  And boxes can
make an impact in one axis but not another (pretty much the definition
of spacing), so is_empty can take an axis as argument now.