Log Message:
Update to 1.5.6, which integrated part of patch-aa (see beta04).

Version 1.5.6beta01 [September 22, 2011]
  Fixed some 64-bit type conversion warnings in pngrtran.c
  Moved row_info from png_struct to a local variable.
  The various interlace mask arrays have been made into arrays of
    bytes and made PNG_CONST and static (previously some arrays were
    marked PNG_CONST and some weren't).
  Additional checks have been added to the transform code to validate the
    pixel depths after the transforms on both read and write.
  Removed some redundant code from pngwrite.c, in png_destroy_write_struct().
  Changed chunk reading/writing code to use png_uint_32 instead of png_byte[4].
    This removes the need to allocate temporary strings for chunk names on
    the stack in the read/write code.  Unknown chunk handling still uses the
    string form because this is exposed in the API.

Version 1.5.6beta02 [September 26, 2011]
  Added a note in the manual the png_read_update_info() must be called only
    once with a particular info_ptr.
  Fixed a typo in the definition of the new PNG_STRING_FROM_CHUNK(s,c) macro.

Version 1.5.6beta03 [September 28, 2011]
  Revised test-pngtest.sh to report FAIL when pngtest fails.
  Added "--strict" option to pngtest, to report FAIL when the failure is
    only because the resulting valid files are different.
  Revised CMakeLists.txt to work with mingw and removed some material from
    CMakeLists.txt that is no longer useful in libpng-1.5.

Version 1.5.6beta04 [October 5, 2011]
  Fixed typo in Makefile.in and Makefile.am ("-M Wl" should be \ 
"-M -Wl")."

Version 1.5.6beta05 [October 12, 2011]
  Speed up png_combine_row() for interlaced images. This reduces the generality
    of the code, allowing it to be optimized for Adam7 interlace.  The masks
    passed to png_combine_row() are now generated internally, avoiding
    some code duplication and localizing the interlace handling somewhat.
  Align png_struct::row_buf - previously it was always unaligned, caused by
    a bug in the code that attempted to align it; the code needs to subtract
    one from the pointer to take account of the filter byte prepended to
    each row.
  Optimized png_combine_row() when rows are aligned. This gains a small
    percentage for 16-bit and 32-bit pixels in the typical case where the
    output row buffers are appropriately aligned. The optimization was not
    previously possible because the png_struct buffer was always misaligned.
  Fixed bug in png_write_chunk_header() debug print, introduced in 1.5.6beta01.

Version 1.5.6beta06 [October 17, 2011]
  Removed two redundant tests for unitialized row.
  Fixed a relatively harmless memory overwrite in compressed text writing
    with a 1 byte zlib buffer.
  Add ability to call png_read_update_info multiple times to pngvalid.c.
  Fixes for multiple calls to png_read_update_info. These fixes attend to
    most of the errors revealed in pngvalid, however doing the gamma work
    twice results in inaccuracies that can't be easily fixed.  There is now
    a warning in the code if this is going to happen.
  Turned on multiple png_read_update_info in pngvalid transform tests.
  Prevent libpng from overwriting unused bits at the end of the image when
    it is not byte aligned, while reading. Prior to libpng-1.5.6 libpng would
    overwrite the partial byte at the end of each row if the row width was not
    an exact multiple of 8 bits and the image is not interlaced.

Version 1.5.6beta07 [October 21, 2011]
  Made png_ptr->prev_row an aligned pointer into png_ptr->big_prev_row
    (Mans Rullgard).

Version 1.5.6rc01 [October 26, 2011]
  Changed misleading "Missing PLTE before cHRM" warning to "Out \ 
of place cHRM"

Version 1.5.6rc02 [October 27, 2011]
  Added LSR() macro to defend against buggy compilers that evaluate non-taken
    code branches and complain about out-of-range shifts.

Version 1.5.6rc03 [October 28, 2011]
  Renamed the LSR() macro to PNG_LSR() and added PNG_LSL() macro.
  Fixed compiler warnings with Intel and MSYS compilers. The logical shift
    fix for Microsoft Visual C is required by other compilers, so this
    enables that fix for all compilers when using compile-time constants.
    Under MSYS 'byte' is a name declared in a system header file, so we
    changed the name of a local variable to avoid the warnings that result.
  Added #define PNG_ALIGN_TYPE PNG_ALIGN_NONE to contrib/pngminim/*/pngusr.h

Version 1.5.6 [November 3, 2011]
  No changes.