Make sure to init PostVS buffers before fetching

* Normally a mesh rendering replay output would do this implicitly when
  an event is selected, but if accessing purely through script this may
  not happen, so we should initialise it ourselves here.
* If it's already init'd, it's almost free to call again.

renderdoc/replay/replay_controller.cpp

@@ -356,6 +356,8 @@ MeshFormat ReplayController::GetPostVSData(uint32_t instID, MeshDataStage stage)
 
   instID = RDCMIN(instID, draw->numInstances - 1);
 
+  m_pDevice->InitPostVSBuffers(draw->eventId);
+
   return m_pDevice->GetPostVSBuffers(draw->eventId, instID, stage);
 }