Consider improvements to progressive loading

[ProgramMax]

A handful of wise image decoding people have commented that PNG's progressive decoding is problematic. For example:

• https://twitter.com/AndreyLipattsev/status/1399302917539180545
• https://twitter.com/jonsneyers/status/1613813170082287617

They're right. This isn't something to consider for 3rd Edition. But it should certainly be considered for future editions.