Add square flag pngs /w genflags.sh script

This uses the github.com:googlei18n/region-flags and imagemagick to generate 27x27 flag pngs. The flags have a 1px #e0e0e0 border and transparent padding such that each flag is of the same height (17px including the border).

res/genflags.sh

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+
+# genflags.sh - Generates pngs for use with CountryDropdown.js
+#
+# Dependencies:
+#   - imagemagick --with-rsvg (because default imagemagick SVG
+#       renderer does not produce accurate results)
+#
+# This will clone the googlei18n flag repo before converting
+# all phonenumber.js-supported country flags (as SVGs) into
+# PNGs that can be used by CountryDropdown.js.
+
+# Allow CTRL+C to terminate the script
+trap "echo Exited!; exit;" SIGINT SIGTERM
+
+# git clone the google repo to get flag SVGs
+git clone git@github.com:googlei18n/region-flags
+for f in region-flags/svg/*.svg; do
+    # Skip state flags
+    if [[ $f =~ [A-Z]{2}-[A-Z]{2,3}.svg ]] ; then
+        echo "Skipping state flag "$f
+        continue
+    fi
+
+    # Skip countries not included in phonenumber.js
+    if [[ $f =~ (AC|CP|DG|EA|EU|IC|TA|UM|UN|XK).svg ]] ; then
+        echo "Skipping non-phonenumber supported flag "$f
+        continue
+    fi
+
+    # Run imagemagick convert
+    #   -background none : transparent background
+    #   -thumbnail 25x15 : resize the flag to have a height of 15.
+    #       By default, aspect ratio is respected so the width will
+    #       be correct and not necessarily 25px.
+    #   -gravity Center  : keep the image central when adding an -extent
+    #   -border 1        : add a 1px border around the flag
+    #   -bordercolor     : set the border colour
+    #   -extent 27x27    : surround the image with padding so that it
+    #       has the dimensions 27x27.
+    convert $f -background none -thumbnail 25x15  \
+    -gravity Center -border 1 -bordercolor \#e0e0e0 \
+    -extent 27x27 $f.png
+
+    # $f.png will be region-flags/svg/XX.svg.png at this point
+
+    # Extract filename from path $f
+    newname=${f##*/}
+    # Replace .svg with .png
+    newname=${newname%.svg}.png
+    # Move the file to flags directory
+    mv $f.png flags/$newname
+    echo "Generated flags/"$newname
+done