[effects/screenedge] Use correct blend function

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}

After:
{F6592392}

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18803

effects/screenedge/screenedgeeffect.cpp

@@ -102,7 +102,7 @@ void ScreenEdgeEffect::paintScreen(int mask, QRegion region, ScreenPaintData &da
         if (effects->isOpenGLCompositing()) {
             GLTexture *texture = (*it)->texture.data();
             glEnable(GL_BLEND);
-            glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+            glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
             texture->bind();
             ShaderBinder binder(ShaderTrait::MapTexture | ShaderTrait::Modulate);
             const QVector4D constant(opacity, opacity, opacity, opacity);