fix: clear flamegraph background in 5.15.12

In QT 5.15.12 the background of the flamegraph is not cleared correctly
which creates phantom entries. This problem does not exists in 5.15.10
and after 5.15.13.

fix: #624

src/flamegraph.cpp

@@ -1188,6 +1188,12 @@ 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)
+    // 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
+
     // 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.
     m_scene->sceneRect();