Allow more leniency

[topin89]

Some images have non-critical errors, like "sBIT" or is too small, having not data for alpha channel or "bKGD" was formed for grayscale, but image itself is in RGBA. Proposed change allows for more leniency so read() function could work on such images.

[drj11]

Do you have any example images?

[drj11]

This is a bit of a slippery slope issue, and I felt the same when I accepted the original PR to add the lenient flag.

The basic problem is "how lenient should we be?".

What are you trying to do, and what have you had to do instead to workaround the problem?

[drj11]

(Mostly notes for my future self)

A possible principle that could be adopted:

• we could be lenient where it does not affect PyPNG's understanding of the PNG file itself.

Under this principle, we could be lenient with a bKGD chunk, because PyPNG doesn't really process it, it just reports it. We could not be lenient with tRNS chunk because PyPNG processes that to create and alpha channel.