Switch to a Copy

This is pretty much just as efficient as all the old overhead was in the TileIndexProperties logic, but means its always a copy and wont have cache mutability problems

Author: SinZ163

src/SMAPI/Framework/Content/AssetDataForMap.cs

@@ -91,7 +91,7 @@ public void PatchMap(Map source, Rectangle? sourceArea = null, Rectangle? target
 
                 // add tilesheet
                 targetSheet = new TileSheet(id, target, sourceSheet.ImageSource, sourceSheet.SheetSize, sourceSheet.TileSize);
-                this.Reflection.GetField<PropertyCollection>(targetSheet, "m_propertyCollection").SetValue((PropertyCollection)sourceSheet.Properties);
+                targetSheet.Properties.CopyFrom(sourceSheet.Properties);
                 target.AddTileSheet(targetSheet);
             }