[scenes/opengl] Fix overlapping shadow tiles

Summary:
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

{F5728514, layout=center, size=full}

Overlapping tiles

{F5728516, layout=center, size=full}

And this is how it supposed to be

{F5728517, layout=center, size=full}

Test Plan:
* apply D11069 to Breeze
* in System Settings/Application Style/Window Decorations, choose "Very Large" shadow size
* open Konsole
* resize it to a minimum possible size

Reviewers: #kwin, graesslin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, ngraham, anemeth, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D10811