go.image/vp8l: use the default code length for repeated codes.
go.image/vp8l: use the default code length for repeated codes.

This fixes the lossless_vec_2_13.webp conformance test.

LGTM=r
R=r, pascal.massimino
CC=golang-codereviews
https://golang.org/cl/143610043

golang/image