D18803: [effects/screenedge] Use correct blend function

zzag created this revision.
zzag added a reviewer: KWin.
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REVISION SUMMARY
  Glow textures already have the red, green, and blue channels
  premultiplied by the alpha component (because of QImage), thus we can't
  use GL_SRC_ALPHA for sfactor in glBlendFunc.
  
  BUG: 403570

TEST PLAN
  Before:
  F6592391: Screenshot_20190207_011841.png <https://phabricator.kde.org/F6592391>
  
  After:
  F6592392: Screenshot_20190207_004853.png <https://phabricator.kde.org/F6592392>

REPOSITORY
  R108 KWin

BRANCH
  screenedge-correct-blend-func

REVISION DETAIL
  https://phabricator.kde.org/D18803

AFFECTED FILES
  effects/screenedge/screenedgeeffect.cpp

To: zzag, #kwin
Cc: kwin, jraleigh, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, ngraham, alexeymin, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart