Add rounded rect intersection tests with difficult rounded rects
where the corners are not disjoint (the 'evil eye').
The first half of these tests were provided by Benjamin Otte
in #6440, the other half was added by me to cover the flipped
version of the evil eye.
These 2 rectangles used to intersect fine:
0 0 50 50 / 50 0
0 0 50 50 / 0 50
But the computed result was:
0 0 50 50 / 50
which is not a valid rectangle, because the corners overlap.
Make sure such rectangles return NOT_REPRESENTABLE.
The above rectangle has been added to the testsuite.
The idea is that for a rectangle intersection, each corner of the
result is either entirely part of one original rectangle or it is
an intersection point.
By detecting those 2 cases and treating them differently, we can
simplify the code to compare rounded rectangles.
