Implement ImGui window for scissor rectangles

This is mainly for debugging, and is only exposed by manually editing the configuration.

Source/Core/VideoCommon/RenderBase.cpp

@@ -627,6 +627,9 @@ void Renderer::DrawDebugText()
   if (g_ActiveConfig.bOverlayProjStats)
     g_stats.DisplayProj();
 
+  if (g_ActiveConfig.bOverlayScissorStats)
+    g_stats.DisplayScissor();
+
   const std::string profile_output = Common::Profiler::ToString();
   if (!profile_output.empty())
     ImGui::TextUnformatted(profile_output.c_str());