Update libpng to version 1.6.3

Change-Id: I86cb46ec376eb41324b82d2875757aeda734c5c1
diff --git a/tests/pngstest-error b/tests/pngstest-error
new file mode 100755
index 0000000..ebf3a7b
--- /dev/null
+++ b/tests/pngstest-error
@@ -0,0 +1,14 @@
+#!/bin/sh
+code=77 # skipped
+for t in "${srcdir}/contrib/pngsuite/"x*".png"
+do
+   if test "$t" != "${srcdir}/contrib/pngsuite/x*.png"
+   then
+      # not skipped, test it
+      if ./pngstest --strict --tmpfile "error" --log "$@" "$t"
+      then
+         code=0 # oops, success: should not happen!
+      fi
+   fi
+done
+exit $code