Make MAX_IFD_NESTING_LEVEL an actual nesting level

Currently we only ever increment ifd_nesting_level, so this ends up
being a limit on the total number of IFD tags and we regularly get

Make MAX_IFD_NESTING_LEVEL an actual nesting level

Currently we only ever increment ifd_nesting_level, so this ends up
being a limit on the total number of IFD tags and we regularly get
bug reports of it being exceeded. I think the intention behind this
limit was to prevent recursion stack overflow, and for that we only
need to check actual recursive usage. I've implemented that here,
and dropped the nesting limit down to a smaller value
(which still passes our tests).

However, it seems that we do also need to have a total limit on
the number of tags, as we don't catch some instances of infinite
looping otherwise. Add this as a separate limit with a higher
value, that should hopefully be sufficient.

This is expected to fix a number of bugs:

https://bugs.php.net/bug.php?id=78083
https://bugs.php.net/bug.php?id=78701
https://bugs.php.net/bug.php?id=79907
https://bugs.php.net/bug.php?id=80016

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