Initialise XdgOutput properties on creation

Summary:
initWaylandOutput can be called before or after all properites are set.
If it's called after, we need to still have xdgOuput set correctly.

Test Plan:
Simplified Aleix's patch that avoids a no-op setScale call
Now it works.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20800

abstract_output.cpp

@@ -171,6 +171,9 @@ void AbstractOutput::createXdgOutput()
         return;
     }
     m_xdgOutput = waylandServer()->xdgOutputManager()->createXdgOutput(m_waylandOutput, m_waylandOutput);
+    m_xdgOutput->setLogicalSize(pixelSize() / scale());
+    m_xdgOutput->setLogicalPosition(m_globalPos);
+    m_xdgOutput->done();
 }
 
 void AbstractOutput::initWaylandOutput()