[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