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
compatibility with the C libwebp implementation.
Current status:
104 PASS, 5 FAIL, 109 TOTAL
Of those 5 failures, 3 are because Go doesn't support lossy-with-alpha
yet, and the other 2 (lossless_vec_2_13.webp,
lossy_extreme_probabilities.webp) look like genuine bugs.
LGTM=r
R=r, pascal.massimino
CC=golang-codereviews
https://golang.org/cl/150000043
