List of features used in each test vector.
All the 'lossless_vec_1_*.webp' WebP files should decode to an image comparable to 'grid.pam' or,
equivalently 'grid.png'.
This synthetic picture is made of 16x16 grid-alternating pixels with RGBA values equal to
blue B=(0,0,255,255) and half-transparent red R=(255,0,0,128), according to
the pattern:
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB
BRBRBRBRBRBRBRBR
RBRBRBRBRBRBRBRB

The 'lossless_vec_2_*.webp' WebP files should decode to an image comparable
to 'peak.pam' or, equivalently 'peak.png'. Their alpha channel is fully
opaque.

Feature list:
lossless_vec_?_0.webp: none
lossless_vec_?_1.webp: PALETTE
lossless_vec_?_2.webp: PREDICTION
lossless_vec_?_3.webp: PREDICTION PALETTE
lossless_vec_?_4.webp: SUBTRACT-GREEN
lossless_vec_?_5.webp: SUBTRACT-GREEN PALETTE
lossless_vec_?_6.webp: PREDICTION SUBTRACT-GREEN
lossless_vec_?_7.webp: PREDICTION SUBTRACT-GREEN PALETTE
lossless_vec_?_8.webp: CROSS-COLOR-TRANSFORM
lossless_vec_?_9.webp: CROSS-COLOR-TRANSFORM PALETTE
lossless_vec_?_10.webp: PREDICTION CROSS-COLOR-TRANSFORM
lossless_vec_?_11.webp: PREDICTION CROSS-COLOR-TRANSFORM PALETTE
lossless_vec_?_12.webp: CROSS-COLOR-TRANSFORM SUBTRACT-GREEN
lossless_vec_?_13.webp: CROSS-COLOR-TRANSFORM SUBTRACT-GREEN PALETTE
lossless_vec_?_14_.webp: PREDICTION CROSS-COLOR-TRANSFORM SUBTRACT-GREEN
lossless_vec_?_15.webp: PREDICTION CROSS-COLOR-TRANSFORM SUBTRACT-GREEN PALETTE