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
