Cache output transformation first

Summary:
First we need to set a transformation, then update the backend. In DRM the
cursor gets updated and needs the new transformed size.

Longterm we need a pending state if we want to apply configuration changes
atomically or deny them.

Test Plan: Compiles.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25903

abstract_wayland_output.cpp

@@ -185,8 +185,8 @@ void AbstractWaylandOutput::applyChanges(const KWayland::Server::OutputChangeSet
     }
     if (changeSet->transformChanged()) {
         qCDebug(KWIN_CORE) << "Server setting transform: " << (int)(changeSet->transform());
-        updateTransform(toTransform(changeSet->transform()));
         setTransform(changeSet->transform());
+        updateTransform(toTransform(changeSet->transform()));
         emitModeChanged = true;
     }
     if (changeSet->positionChanged()) {