[scenes/opengl] Print a debug message when viewport limits aren't met

Summary:
This may help with debugging why compositing is suspended.

CCBUG: 418951

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28139

plugins/scenes/opengl/scene_opengl.cpp

@@ -800,6 +800,7 @@ bool SceneOpenGL::viewportLimitsMatched(const QSize &size) const {
     if (limit[0] < size.width() || limit[1] < size.height()) {
         auto compositor = static_cast<X11Compositor*>(Compositor::self());
         QMetaObject::invokeMethod(compositor, [compositor]() {
+            qCDebug(KWIN_OPENGL) << "Suspending compositing because viewport limits are not met";
             compositor->suspend(X11Compositor::AllReasonSuspend);
         }, Qt::QueuedConnection);
         return false;