learnDestroy(type, callback) {
        this.destructors[type] = callback;
    }
Teach the system to destroy data type 'type'
for example, textures loaded to GPU have to be also manually removed when not needed anymore.
Needs to be defined only when the created object has extra deletion process.
@param {string} type
@param {OpenSeadragon.TypeDestructor} callback destructor, receives the object created,
  it is basically a type conversion to 'undefined' - thus the type.