Issue #169: Init X11 threads for threaded buffer swap

Ref: http://blog.qt.io/blog/2011/06/03/threaded-opengl-in-4-8/

src/mcedit2/main.py

@@ -157,6 +157,8 @@ editorApp = None
 
 @profiler.function("startup")
 def startup():
+    # Must call this before QApplication is created
+    QtCore.QCoreApplication.setAttribute(QtCore.Qt.AA_X11InitThreads, True)
     global editorApp
     setup_logging()
     sys.excepthook = excepthook