[plugins/scenes/opengl] Use linear filter on Wayland

Summary:
With nearest filter fractional scaling is blurry, always use linear filter
instead on Wayland.

Test Plan: Run Plasma session with patch and scale factor 1.9.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T10481

Differential Revision: https://phabricator.kde.org/D23986