Add bash scripts to check .png files for libpng errors

tools/png-verify.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# Try to open .png files and display libpng warning if any.
+
+if [ "$#" -eq 0 ]; then
+    find ../addons-src/ data/ -iname "*.png" -print0 | while read -d $'\0' i; do echo "$(pngtopnm "$i" 2>&1 > /dev/null )" | sed "s#^#${i}: #"; done | grep -v ': $'
+else
+  for i in "$@"; do
+    pngtopnm "$i" 2>&1 > /dev/null | sed "s#^#${i}: #" | grep -v ': $'
+  done
+fi
+
+# EOF #