Serialize cubeTexture matrix so that cloning works properly

src/Materials/Textures/cubeTexture.ts

@@ -81,7 +81,10 @@ export class CubeTexture extends BaseTexture {
     private _noMipmap: boolean;
     private _files: string[];
     private _extensions: string[];
+
+    @serialize("textureMatrix")
     private _textureMatrix: Matrix;
+
     private _format: number;
     private _createPolynomials: boolean;