Restore the backend set viewport when popping the final render target

Summary:
The backend is responsible for setting the viewport, the GLRenderTarget
should save and restore the viewport actually set by the backend.

Test Plan:
Ran Blur effect which uses this method, under scaling the viewport "restored"
differed from the one set by the backend.

Reviewers: Plasma

Subscribers: plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4952