Log Message:
Update to 1.6.0:

This release adds a "simplified API" and removes some features
that have been deprecated since libpng.1.5.x.

Changes to Libpng from version 1.5.x to 1.6.x:

A "simplified API" has been added (see documentation in png.h and a simple
example in contrib/examples/pngtopng.c).  The new publicly visible API
includes the following:

   macros:
     PNG_FORMAT_*
     PNG_IMAGE_*
   structures:
     png_control
     png_image
   read functions
     png_image_begin_read_from_file()
     png_image_begin_read_from_stdio()
     png_image_begin_read_from_memory()
     png_image_finish_read()
     png_image_free()
   write functions
     png_image_write_to_file()
     png_image_write_to_stdio()

Starting with libpng-1.6.0, you can configure libpng to prefix all exported
symbols, using the PNG_PREFIX macro.

We no longer include string.h in png.h.  The include statement has been moved
to pngpriv.h, where it is not accessible by applications.  Applications that
need access to information in string.h must add an '#include "string.h"'
directive.  It does not matter whether this is placed prior to or after
the '"#include png.h"' directive.

The following API are now DEPRECATED:
   png_info_init_3()
   png_convert_to_rfc1123() which has been replaced
     with png_convert_to_rfc1123_buffer()
   png_data_freer()
   png_malloc_default()
   png_free_default()
   png_reset_zstream()

The following has been removed:
   png_get_io_chunk_name(), which has been replaced
     with png_get_io_chunk_type().  The new
     function returns a 32-bit integer instead of
     a string.
   The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
     png_memset() macros are no longer used in the libpng sources and
     have been removed.  These had already been made invisible to
applications
     (i.e., defined in the private pngpriv.h header file) since
libpng-1.5.0.

The signatures of many exported functions were changed, such that
   png_structp became png_structrp or png_const_structrp
   png_infop became png_inforp or png_const_inforp
where "rp" indicates a "restricted pointer".

Error detection in some chunks has improved; in particular the iCCP chunk
reader now does pretty complete validation of the basic format.  Some bad
profiles that were previously accepted are now rejected, in particular the
very old broken Microsoft/HP sRGB profile.

The library now issues a warning if both background processing and RGB to
gray are used when gamma correction happens. As with previous versions of
the library the results are numerically very incorrect in this case.

There are some minor arithmetic changes in some transforms such as
png_set_background(), that might be detected by certain regression tests.

Unknown chunk handling has been improved internally, without any API change.
This adds more correct option control of the unknown handling, corrects
a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes
it possible to skip IDAT chunks in the sequential reader.