Backport 'tagged' render invokes from qrenderdoc

* This is a possible fix for a case where render work triggered by mouse
  movements (such as pixel and vertex picking) can happen faster than it
  executes, leading to a backlog of render commands and a noticeable lag
  which only gets worse the more you move the mouse until everything
  seems to be unresponsive or laggy (especially if you then trigger a
  blocking command like event change, which will block the whole UI
  until the queued picks happen).
* Since a new pick coming in will override and make redundant the
  previous pick, we allow the render commands to do just that. If a new
  command comes in, we remove any previous commands with the same tag
  and put the command in the first match (this prevents a tagged invoke
  always being pushed to the back of the queue).

renderdocui/Controls/ResourcePreview.cs

@@ -155,7 +155,7 @@ namespace renderdocui.Controls
             }
 
             if (m_Output != null)
-                m_Core.Renderer.InvokeForPaint((ReplayRenderer r) => { m_Output.Display(); });
+                m_Core.Renderer.InvokeForPaint("thumbpaint", (ReplayRenderer r) => { m_Output.Display(); });
         }
 
         public void SetSize(Size s)