kwin: Initialize QElapsedTimers by calling invalidate()

QElapsedTimer doesn't have a constructor that initializes the
private members. This means that calls to isValid() will depend
on uninitialized data when the timer has never been started.

elapsed() and restart() will also use and return values
computed from uninitialized data.

This fixes several valgrind errors.

scene.cpp

@@ -98,6 +98,7 @@ Scene::Scene(Workspace* ws)
     , lanczos_filter(new LanczosFilter())
     , m_overlayWindow(new OverlayWindow())
 {
+    last_time.invalidate(); // Initialize the timer
 }
 
 Scene::~Scene()

workspace.cpp

@@ -222,6 +222,8 @@ Workspace::Workspace(bool restore)
     // need to create the tabbox before compositing scene is setup
     tab_box = new TabBox::TabBox(this);
 #endif
+
+    nextPaintReference.invalidate(); // Initialize the timer
     setupCompositing();
 
     // Compatibility