For a block invoke to another thread, safe and restore

* In future we could handle async exceptions by storing the exception
  information in a std::function derived object (instead of the separate
  ExceptionHandling that lives on the stack) and query it out in a new
  WaitForInvoke function maybe. Right now we just print the exception
  to the output log and abort the callback.

qrenderdoc/Code/pyrenderdoc/PythonContext.h

@@ -89,6 +89,8 @@ public:
   }
   static QWidget *QWidgetFromPy(PyObject *widget);
 
+  void setThreadBlocking(bool block) { m_Block = block; }
+  bool threadBlocking() { return m_Block; }
   void abort() { m_Abort = true; }
   bool shouldAbort() { return m_Abort; }
   QString currentFile() { return location.file; }
@@ -128,6 +130,8 @@ private:
 
   bool m_Abort = false;
 
+  bool m_Block = false;
+
   static PyObject *QtObjectToPython(PyObject *self, const char *typeName, QObject *object);
 
   QTimer *outputTicker;