these parts of the unit tests seem to use a random number generator with no
fixed seed, which can cause sporadic test failures (e.g.
https://api.travis-ci.com/v3/job/357101203/log.txt):

./image/tests/test_resampling.py:47:    data = np.random.randint(0, 10, shape)
./image/tests/test_resampling.py:223:    data = np.random.randint(0, 10, shape)
./image/tests/test_resampling.py:556:    so

Introduce a decode flag that produces a vertically flipped image, this would only be valid in non-progressive mode.