[plugins/qpa] Commit wl_surface when flipping FBOs

Summary:
Even though the buffer is passed through a side channel some properties
we might use are double-buffered on the surface commit.

Test Plan: Used by unit test InternalWindowTest::testScale in linked commit

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18085