recompress screenshots using zopflipng, strip using exiftool

This makes them as small as possible.

tools/png-pre-compress

@@ -7,5 +7,12 @@ for f in `find $(dirname $0)/../app/src/ -type f -name \*.png`; do
     echo $f | grep -Eo '\.9\.png$' && continue  # do not optimized 9-patch, it breaks them
     tmpfile=$(mktemp)
     aapt singleCrunch -v -i $f -o $tmpfile
+    exiftool -all= $tmpfile
     mv $tmpfile $f
 done
+
+for f in metadata/*/images/*Screenshots/*.png; do
+    exiftool -all= $f
+    tmpfile=$(mktemp)
+    (zopflipng --filters=01234mepb --lossy_8bit --lossy_transparent -y $f $tmpfile && mv $tmpfile $f) &
+done