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 MSVC warnings

• Move transparency testing for grayscale images out of the decoding loop, move it to trns_row()

• Use get_sample() to iterate over 1/2/4-bit samples everywhere

• Update check_png_keyword() before something bad happens

• Use memcpy() less, omit where possible