[libpng16] Allow fine grain control of unknown chunk APIs. This change allows
png_set_keep_unknown_chunks() to be turned off if not required and causes
both read and write to behave appropriately (on read this is only possible
if the user callback is used to handle unknown chunks). The change
also removes the support for storing unknown chunks in the info_struct
if the only unknown handling enabled is via the callback, allowing libpng
to be configured with callback reading and none of the unnecessary code.
7 files changed
