This fixes all three lossy_alpha*.webp conformance tests.
The test data was generated by cwebp/dwebp version 0.4.1:
cwebp yellow_rose.png -o yellow_rose.lossy-with-alpha.webp
dwebp yellow_rose.lossy-with-alpha.webp -pgm -o tmp.pgm
convert tmp.pgm yellow_rose.lossy-with-alpha.webp.nycbcra.png
LGTM=pascal.massimino, r
R=r, pascal.massimino
CC=golang-codereviews
https://golang.org/cl/154350043
