D14784: [scenes/opengl] Fix overlaps in shadow texture atlas

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REVISION SUMMARY
  If the corner shadow tiles(top-left, top-right, and so) 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: tooltip-before.png <https://phabricator.kde.org/F6190484>
  
  After:
  F6190488: tooltip-after.png <https://phabricator.kde.org/F6190488>
  
  Depends on D14783 <https://phabricator.kde.org/D14783>

REPOSITORY
  R108 KWin

BRANCH
  scenes-opengl-missing-shadow-corner-tiles

REVISION DETAIL
  https://phabricator.kde.org/D14784

AFFECTED FILES
  autotests/integration/scene_opengl_shadow_test.cpp
  plugins/scenes/opengl/scene_opengl.cpp

To: zzag, #kwin
Cc: kwin, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart