[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:

After:

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

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