[libpng16] Updated pngvalid.c with recent changes from libpng17

ANNOUNCE

@@ -1,4 +1,4 @@
-Libpng 1.6.19beta03 - September 15, 2015
+Libpng 1.6.19beta03 - September 18, 2015
 
 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.
@@ -64,8 +64,19 @@ Version 1.6.19beta02 [August 19, 2015]
     in ANSI-C (unlike 0x80000000 in the signed case) the checking that
     occurs later can catch them (John Bowler).
 
-Version 1.6.19beta03 [September 15, 2015]
+Version 1.6.19beta03 [September 18, 2015]
   Fixed png_save_int_32 when int is not 2's complement (John Bowler).
+  Updated libpng16 with all the recent test changes from libpng17,
+    including changes to pngvalid.c to ensure that the original,
+    distributed, version of contrib/visupng/cexcept.h can be used
+    (John Bowler).
+  pngvalid contains the correction to the use of SAVE/STORE_
+    UNKNOWN_CHUNKS; a bug revealed by changes in libpng 1.7. More
+    tests contain the --strict option to detect warnings and the
+    pngvalid-standard test has been corrected so that it does not
+    turn on progresive-read. There is a separate test which does
+    that. (John Bowler)
+  Also made some signed/unsigned fixes.
 
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