fix: Prevent unknown chunks from causing out-of-place IEND errors

PNG_AFTER_IDAT was not set by the IDAT read code if unknown chunk
handling was turned on.  This was hidden in the current tests by checks
within the text handling chunks. (For example, pngtest.png has a zTXt
chunk after IDAT.)

This change modifies both the sequential and the progressive reader to
reliably set PNG_AFTER_IDAT when the first non-IDAT chunk is seen and
before that chunk is processed.

The change is minimalist; PNG_HAVE_CHUNK_AFTER_IDAT can probably be
removed and replaced with PNG_AFTER_IDAT.  Making the latter change is
something to be considered in libpng2.

Co-authored-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>