Summary:
If the corner shadow tiles(top-left, top-right, and so on) tiles are missing,
then the left/top/right/bottom shadow tiles will overlap.

This diff addresses that problem by changing how the shadow texture
atlas is rendered:
* corner tiles will be drawn in the corners of the atlas(buildQuads
  method expects them to be at the corners);
* top, right, bottom, and left tile will be aligned to the top-left
  corner of the inner shadow rect.

For majority of desktop themes, the shadow texture atlas looks the same.
For example, here's for Aether:

Before:
{F6190484, layout=center, size=full}

After:
{F6190488, layout=center, size=full}

Depends on D14783

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14784