Fix viewport restore in GLRenderTarget::popRenderTarget

Summary:
The viewport needs adjustment in the per-output rendering case. This
change ensures the viewport is setup like in the platforms which do per
output rendering. For the X11 case (multiple outputs in one render pass)
the values are the same as previously.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3099