Destroy materials and properties when MPC is destroyed

Source/ConformalDecals/MaterialModifiers/MaterialPropertyCollection.cs

@@ -99,6 +99,15 @@ namespace ConformalDecals.MaterialModifiers {
             _materialProperties ??= new Dictionary<string, MaterialProperty>();
         }
 
+        public void OnDestroy() {
+            if (_decalMaterial != null) Destroy(_decalMaterial);
+            if (_previewMaterial != null) Destroy(_previewMaterial);
+
+            foreach (var entry in _materialProperties) {
+                Destroy(entry.Value);
+            }
+        }
+
         public void AddProperty(MaterialProperty property) {
             if (property == null) throw new ArgumentNullException(nameof(property));