fix: broken flamegraph

Apparently when running X11 the flamgraph doesn't clear the background
correctly. This patch resets the background brush which clears the
background correctly.

fixes: #624

src/flamegraph.cpp

@@ -1189,11 +1189,11 @@ void FlameGraph::selectItem(FrameGraphicsItem* item)
     // then layout all items below the selected on
     layoutItems(item);
 
-#if QT_VERSION <= QT_VERSION_CHECK(5, 15, 12) && QT_VERSION > QT_VERSION_CHECK(5, 15, 10)
+    // #if QT_VERSION <= QT_VERSION_CHECK(5, 15, 12) && QT_VERSION > QT_VERSION_CHECK(5, 15, 10)
     // the scene background doesn't get cleared correctly when using qt 5.15.12
     // this doesn't happen in 5.15.10 and 5.15.13
     m_scene->setBackgroundBrush(QBrush());
-#endif
+    // #endif
 
     // Triggers a refresh of the scene's bounding rect without going via the
     // event loop. This makes the centerOn call below work as expected in all cases.