Implement ImGui window for scissor rectangles

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

Source/Core/VideoCommon/VertexShaderManager.cpp

@@ -299,6 +299,7 @@ void VertexShaderManager::SetConstants()
 
     dirty = true;
     BPFunctions::SetScissorAndViewport();
+    g_stats.AddScissorRect();
   }
 
   if (bProjectionChanged || g_freelook_camera.GetController()->IsDirty())