gc() {
        this.freeUnusedTextureSources();
        this._cleanupLookupMap(); // we are calling the function here
    }
    
    freeUnusedTextureSources() {
        let remainingTextureSources = [];
        for (let i = 0, n = this._uploadedTextureSources.length; i < n; i++) {
            let ts = this._uploadedTextureSources[i];
            if (ts.allowCleanup()) {
                this._freeManagedTextureSource(ts);
            } else {
                remainingTextureSources.push(ts);
            }
        }

        this._uploadedTextureSources = remainingTextureSources;

        this._cleanupLookupMap(); // and we already call it here
    }