box: Flip the intersection check around

We use the same notation as the equivalent graphene_rect_intersection()
function, but we also need to use the cmp_ge() SIMD operator because
both the SIMD operands have the fourth channel set to 0. Using cmp_le()
would introduce false positives.
box: Re-use the API inside the implementation

This increases the code coverage, and reduces the chances of getting
things wrong.
tests/box: Add more units
box: Flip the contains checks around

Just like we did in commit e8038beb5d3b3466387ffbda9594fe63c553389e, we
need to flip the comparison operators in the contains_point() and
contains_box() methods to avoid false negatives.
tests/box: Add more units

Improve the test suite coverage for graphene_box_t.