Fix Python callback reference counting and shutdown cleanup

Clear the argument tuple after calling a Python callback so varfunc's
destructor cannot decrement it a second time. This prevents repeated
zero-argument callbacks from losing references to the shared empty tuple.

Skip callback reference cleanup after Python has shut down, since Qt can
destroy pending callback closures during QApplication teardown.

Validation:
- Regression harness using this header and bundled Python 3.6.4: 20,000
  callbacks with balanced references, nonempty arguments, exception paths,
  deferred release, and callback destruction after Py_Finalize.
- Windows x64 Release build: no warnings or errors.
- MI 8 remote replay: 1.rdc -> 2.rdc -> 1.rdc -> 2.rdc; normal exit with
  a pending callback, exit code 0.

Build and device checks used the existing development checkout, which also
contains separate local changes excluded from this commit.

qrenderdoc/Code/pyrenderdoc/function_conversion.h

@@ -184,6 +184,10 @@ struct PyObjectRefCounter
   PyObjectRefCounter(const PyObjectRefCounter &o) { obj = Py_NewRef(o.obj); }
   ~PyObjectRefCounter()
   {
+    // Qt can release pending callbacks after Python has shut down.
+    if(!Py_IsInitialized())
+      return;
+
     // it may not be safe at the point this is destroyed to decref the object. For example if a
     // python lambda is passed into a C++ invoke function, we will be holding the only reference to
     // that lambda here when the async invoke completes and destroyed the std::function wrapping it.
@@ -249,7 +253,7 @@ struct varfunc
 
     PyObject *result = DoFunctionCall(func, args);
 
-    Py_DECREF(args);
+    Py_CLEAR(args);
 
     if(result == NULL)
     {