[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

plugins/qpa/window.cpp

@@ -148,6 +148,7 @@ QSharedPointer<QOpenGLFramebufferObject> Window::swapFBO()
 {
     auto fbo = m_contentFBO;
     m_contentFBO.clear();
+    m_surface->commit(KWayland::Client::Surface::CommitFlag::None);
     return fbo;
 }