Disable GraphicsScene indexing method to prevent crashes

Under certain circumstances Carla would crash when frequent updates on
the QGraphicsScene occurred, especially in combination with scrolling or
panning by the user. Backtraces showed either a segfault in
QGraphicsItem::parentItem() or around
QGraphicsSceneBspTree::climbTree(). While the first trace points towards
missing calls to pepareGeometryChange() in combination with custom
boundingRect() methods, inserting these did not yield a positive change,
though people with this issue also had success in disabling the
ItemIndexingMethod of their QGraphicsScene[0][1]. Setting this to
NoIndex solves this problem.

The original problem is reproducible with an ever-reconnecting client:

$ while true; do klick 120 & (sleep 0.1; killall klick); done

[0] https://bugreports.qt.io/browse/QTBUG-18021?focusedCommentId=367566&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-367566
[1] https://forum.qt.io/topic/71316/qgraphicsscenefinditembsptreevisitor-visit-crashes-due-to-an-obsolete-paintevent-after-qgraphicsscene-removeitem

source/frontend/patchcanvas/scene.py

@@ -81,6 +81,7 @@ class PatchScene(QGraphicsScene):
         self.m_cursor_cut = None
         self.m_cursor_zoom = None
 
+        self.setItemIndexMethod(QGraphicsScene.ItemIndexMethod.NoIndex)
         self.selectionChanged.connect(self.slot_selectionChanged)
 
     def getDevicePixelRatioF(self):