This problem appears if shadow corner tiles are too big and
some window has size smaller than 2 * shadowTileSize.
This change tries to address the problem above by exclusing
overlapping tile parts. If there are any two overlapping corners
then tile between them(top/right/bottom/left) is not rendered.
Also, because some corner tile parts can be excluded, corner tiles
are expected to be symmetrical(i.e. if we remove right half from
the top-left tile and left half from the top-right tile and
stick them together, they still look fine, there are no misalignments, etc).
Most shadows(e.g. shadows from Breeze) have such behaviour.
No tiles are overlapping
And this is how it supposed to be