Fix crash due to dangling reference

Summary:
Returning a reference to a value on the stack is broken.
This caused kwin_wayland to crash in openQA with a nullptr deref.

BUG: 407199

Test Plan: Only build tested.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21085

plugins/platforms/drm/drm_output.cpp

@@ -144,7 +144,7 @@ int orientationToRotation(Qt::ScreenOrientation orientation)
     return 0;
 }
 
-QMatrix4x4 &&DrmOutput::matrixDisplay(const QSize &s) const
+QMatrix4x4 DrmOutput::matrixDisplay(const QSize &s) const
 {
     QMatrix4x4 matrix;
     const int angle = orientationToRotation(orientation());
@@ -156,7 +156,7 @@ QMatrix4x4 &&DrmOutput::matrixDisplay(const QSize &s) const
         matrix.translate(-center.width(), -center.height());
     }
     matrix.scale(scale());
-    return std::move(matrix);
+    return matrix;
 }
 
 void DrmOutput::updateCursor()

plugins/platforms/drm/drm_output.h

@@ -135,7 +135,7 @@ private:
 
     int getGammaRampSize() const override;
     bool setGammaRamp(const ColorCorrect::GammaRamp &gamma) override;
-    QMatrix4x4 &&matrixDisplay(const QSize &s) const;
+    QMatrix4x4 matrixDisplay(const QSize &s) const;
 
     DrmBackend *m_backend;
     DrmConnector *m_conn = nullptr;