Copy a stored block, storing first the length and its one's complement if requested.
function copy_block(buf, // the input data
		len, // its length
		header // true if block header must be written
		) {
			bi_windup(); // align on byte boundary
			last_eob_len = 8; // enough lookahead for inflate

			if (header) {
				put_short(len);
				put_short(~len);
			}

			that.pending_buf.set(window.subarray(buf, buf + len), that.pending);
			that.pending += len;
		}