修改 fbx 默认材质

防止 Standard shader 被错误打进包中，需要自行创建项目的默认材质

public class FbxImport : AssetPostprocessor
{
	private Material OnAssignMaterialModel(Material material, Renderer renderer)  
	{  
		return AssetDatabase.LoadAssetAtPath<Material>("Assets/Res/Mat/Default_Mat.mat");  
	}
}