Check libpng directly into third_party/

third_party/libpng/ANNOUNCE

+Libpng 1.6.22rc01 - May 14, 2016
+
+This is not intended to be a public release.  It will be replaced
+within a few weeks by a public version or by another test version.
+
+Files available for download:
+
+Source files with LF line endings (for Unix/Linux) and with a
+"configure" script
+
+   1.6.22rc01.tar.xz (LZMA-compressed, recommended)
+   1.6.22rc01.tar.gz
+
+Source files with CRLF line endings (for Windows), without the
+"configure" script
+
+   lp1622r01.7z  (LZMA-compressed, recommended)
+   lp1622r01.zip
+
+Other information:
+
+   1.6.22rc01-README.txt
+   1.6.22rc01-LICENSE.txt
+   libpng-1.6.22rc01-*.asc (armored detached GPG signatures)
+
+Changes since the last public release (1.6.21):
+
+Version 1.6.22beta01 [January 23, 2016]
+  Changed PNG_USE_MKSTEMP to __COVERITY__ to select alternate
+    "tmpfile()" implementation in contrib/libtests/pngstest.c
+  Fixed NO_STDIO build of pngunknown.c to skip calling png_init_io()
+    if there is no stdio.h support.
+  Added a png_image_write_to_memory() API and a number of assist macros
+    to allow an application that uses the simplified API write to bypass
+    stdio and write directly to memory.
+  Added some warnings (png.h) and some check code to detect *possible*
+    overflow in the ROW_STRIDE and simplified image SIZE macros.  This
+    disallows image width/height/format that *might* overflow.  This is
+    a quiet API change that limits in-memory image size (uncompressed) to
+    less than 4GByte and image row size (stride) to less than 2GByte.
+  Revised workaround for false-positive Coverity issue in pngvalid.c.
+
+Version 1.6.22beta02 [February 8, 2016]
+  Only use exit(77) in configure builds.
+  Corrected error in PNG_IMAGE_PNG_SIZE_MAX. This new macro underreported
+    the palette size because it failed to take into account that the memory
+    palette has to be expanded to full RGB when it is written to PNG.
+  Updated CMakeLists.txt, added supporting scripts/gen*.cmake.in
+    and test.cmake.in (Roger Leigh).
+  Relaxed limit checks on gamma values in pngrtran.c. As suggested in
+    the comments gamma values outside the range currently permitted
+    by png_set_alpha_mode are useful for HDR data encoding.  These values
+    are already permitted by png_set_gamma so it is reasonable caution to
+    extend the png_set_alpha_mode range as HDR imaging systems are starting
+    to emerge.
+
+Version 1.6.22beta03 [March 9, 2016]
+  Added a common-law trademark notice and export control information
+    to the LICENSE file, png.h, and the man page.
+  Restored "& 0xff" in png_save_uint_16() and png_save_uint_32() that
+    were accidentally removed from libpng-1.6.17. 
+  Changed PNG_INFO_cHNK and PNG_FREE_cHNK from 0xnnnn to 0xnnnnU in png.h
+    (Robert C. Seacord).
+  Removed dubious "#if INT_MAX" test from png.h that was added to
+    libpng-1.6.19beta02 (John Bowler).
+  Add ${INCLUDES} in scripts/genout.cmake.in (Bug report by Nixon Kwok).
+  Updated LICENSE to say files in the contrib directory are not
+    necessarily under the libpng license, and that some makefiles have
+    other copyright owners.
+  Added INTEL-SSE2 support (Mike Klein and Matt Sarett, Google, Inc.).
+  Made contrib/libtests/timepng more robust.  The code no longer gives
+    up/fails on invalid PNG data, it just skips it (with error messages).
+    The code no longer fails on PNG files with data beyond IEND.  Options
+    exist to use png_read_png (reading the whole image, not by row) and, in
+    that case, to apply any of the supported transforms.  This makes for
+    more realistic testing; the decoded data actually gets used in a
+    meaningful fashion (John Bowler).
+  Fixed some misleading indentation (Krishnaraj Bhat).
+
+Version 1.6.22beta04 [April 5, 2016]
+  Force GCC compilation to C89 if needed (Dagobert Michelsen).
+  SSE filter speed improvements for bpp=3:
+    memcpy-free implementations of load3() / store3().
+    call load3() only when needed at the end of a scanline.
+
+Version 1.6.22beta05 [April 27, 2016]
+  Added PNG_FAST_FILTERS macro (defined as
+    PNG_FILTER_NONE|PNG_FILTER_SUB|PNG_FILTER_UP).
+  Various fixes for contrib/libtests/timepng.c
+  Moved INTEL-SSE code from pngpriv.h into contrib/intel/intel_sse.patch.
+  Fixed typo (missing underscore) in #define PNG_READ_16_TO_8_SUPPORTED
+    (Bug report by Y.Ohashik).
+
+Version 1.6.22beta06 [May 5, 2016]
+  Rebased contrib/intel_sse.patch.
+  Quieted two Coverity issues in contrib/libtests/timepng.c.
+  Fixed issues with scripts/genout.cmake.in (David Capello, Nixon Kwok):
+    Added support to use multiple directories in ZLIBINCDIR variable,
+    Fixed CMAKE_C_FLAGS with multiple values when genout is compiled on MSVC,
+    Fixed pnglibconf.c compilation on OS X including the sysroot path.
+
+Version 1.6.22rcp1 [May 14, 2016]
+  No changes.
+
+Send comments/corrections/commendations to png-mng-implement at lists.sf.net
+(subscription required; visit
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+to subscribe)
+or to glennrp at users.sourceforge.net
+
+Glenn R-P