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

To-do list for post-v0.6.0:

• read_chunk_bytes() - simplify the crc_action check

• Move the "discard trailing IDATs" stage to read_chunks()

• Use an internal struct spng_text2 with just a pointer for the keyword to simplify parsing

• Clean up spng_decode_scanline() (e.g. use a ihdr pointer)

• Clean up flag handling in spng_decode_image()

• Fix