Add explicit functions to initialise and shutdown replay. Closes #1685

* It's required for replay applications to call InitialiseReplay() before doing
  any work, and to call ShutdownReplay() once they're finished.
* This lets us do more heavyweight shutdown work before global destructors are
  being invoked and the RenderDoc instance is being destroyed.
* Anything that needs to be shut down during capture still has to happen in
  RenderDoc::~RenderDoc since we obviously can't get the application to call a
  shutdown function in that situation.

qrenderdoc/Code/qrenderdoc.cpp

@@ -378,7 +378,7 @@ int main(int argc, char *argv[])
         coreargs.push_back("--crash");
       for(const QString &arg : remaining)
         coreargs.push_back(arg);
-      RENDERDOC_InitGlobalEnv(env, coreargs);
+      RENDERDOC_InitialiseReplay(env, coreargs);
     }
 
     if(!crashReportPath.isEmpty())
@@ -490,6 +490,8 @@ int main(int argc, char *argv[])
       config.Save();
     }
 
+    RENDERDOC_ShutdownReplay();
+
     PythonContext::GlobalShutdown();
 
     Formatter::shutdown();