Convert every flags to png and optimize them

Only saves from 200 to 300 bytes per image, but it improves consistency.

Used convert from imagemagick and optipng
```sh
#convert to png
find . -name '*.gif' | while read line; do echo $line; convert "$line" "$(echo $line | sed 's/\.gif/.png/g')"; done
#optimize
optipng -o7 *.png
```