[backends/hwcomposer] Blank output on tear down

This gives a better tear down experience as it goes to black instead
of showing outdated screen and also it disables vsync which fixes a
crash on teardown.

backends/hwcomposer/hwcomposer_backend.cpp

@@ -47,6 +47,9 @@ HwcomposerBackend::HwcomposerBackend(QObject *parent)
 
 HwcomposerBackend::~HwcomposerBackend()
 {
+    if (!m_outputBlank) {
+        toggleBlankOutput();
+    }
     if (m_device) {
         hwc_close_1(m_device);
     }