public Builder setConfig(Bitmap.Config config) {
            this.config = config;
            return this;
        }
Sets the {@link Bitmap.Config} for the Bitmaps to pre-fill.
@param config The config to use, or null to use Glide's default.
@return This builder.
core/src/main/java/com/bumptech/glide/load/engine/prefill/PreFillType.java