add exhaustive test vectors for lossless coding

A simple 'test_lossless.sh' shell script is supplied for testing
with 'dwebp'.

See lossless_vec_list.txt for details of the features and images used
to generate the files.

Basically:
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

Change-Id: I36dc37c63cb717349cc5ede6a0e4824a313212b2