Invoke CaptureContext function calls onto UI thread, from python thread

* Any work that might use Qt needs to happen on the UI thread, so when
  running a python script on the python thread we need to invoke across.
  We wrap the main ICaptureContext interface to block invoke onto the UI
  for any function calls that aren't just returning internal data.

qrenderdoc/Windows/PythonShell.h

@@ -36,6 +36,8 @@ namespace Ui
 class PythonShell;
 }
 
+struct CaptureContextInvoker;
+
 class PythonShell : public QFrame, public IPythonShell
 {
   Q_OBJECT
@@ -66,6 +68,7 @@ private slots:
 private:
   Ui::PythonShell *ui;
   ICaptureContext &m_Ctx;
+  CaptureContextInvoker *m_ThreadCtx = NULL;
 
   ScintillaEdit *scriptEditor;