[scenes/opengl] Set correct blend mode for EffectQuickView buffers

Summary:
Everything is already multiplied in the buffer, we want to copy the
source directly.

Test Plan:
Looked at something with EffectQuickView and alpha
It now matched what it should be

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24368

plugins/scenes/opengl/scene_opengl.cpp

@@ -874,8 +874,7 @@ void SceneOpenGL::paintEffectQuickView(EffectQuickView *w)
     shader->setUniform(GLShader::ModelViewProjectionMatrix, mvp);
 
     glEnable(GL_BLEND);
-    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
-
+    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
     t->bind();
     t->render(QRegion(infiniteRegion()), w->geometry());
     t->unbind();