Add new Big Sur-style rectangle icon for macOS

Fixes #1253

scripts/generate-icon-macos.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+set -e
+cd "$(dirname "$0")/.."
+
+if [ "$(uname -s)" != Darwin ]; then
+    echo "This script only runs on macOS since it needs 'iconutil'"
+    exit 1
+fi
+
+if ! command -v inkscape &> /dev/null; then
+    echo "Please make sure to have 'inkscape' on your PATH!"
+    exit 1
+fi
+
+input_svg="build/icon-macos.svg"
+tmp_dir="$(mktemp -dt zulip-icon)"
+output_dir="$tmp_dir.iconset"
+output_icns="build/icon.icns"
+
+mv "$tmp_dir" "$output_dir"
+
+trap "rm -rf '$output_dir'" EXIT
+
+echo "==> Generating icons from $input_svg..."
+
+for size in 16 32 64 128 256 512 1024; do
+    echo "Generating icon of size $size..."
+    icon_prefix="icon_${size}x${size}"
+    inkscape -o "$output_dir/$icon_prefix.png"    -w $size         "$input_svg"
+    inkscape -o "$output_dir/$icon_prefix@2x.png" -w $((size * 2)) "$input_svg"
+done
+
+echo "==> Updating ICNS icons..."
+
+for icns in "${output_icns[@]}"; do
+    echo "Updating $icns..."
+    iconutil -c icns -o "$icns" "$output_dir"
+done