Fix C++ invokes being responsible for destroying python callbacks

* If the last refcount on a python lambda/temp function is released when a
  wrapping std::function is destroyed in a C++ invoke, we can't destroy it
  safely. Instead we queue up that decref and process it the next chance we're
  able (which is either when the current execution finishes for a python shell
  execution, or on the next function call which handles extensions).

qrenderdoc/Code/pyrenderdoc/pyrenderdoc_stub.cpp

@@ -64,4 +64,12 @@ extern "C" void SetThreadBlocking(PyObject *global_handle, bool block)
 {
 }
 
+extern "C" void QueueDecRef(PyObject *obj)
+{
+}
+
+extern "C" void ProcessDecRefQueue()
+{
+}
+
 REPLAY_PROGRAM_MARKER()