Meta: Add a PNG size check to CI and pre-commit checks

This uses optipng to check how much size can be reduced on PNG files. If
that's more than 2 KiB for at least one file, the check fails. As with
other checks, it doesn't run if optipng is not installed.

Meta/check-png-sizes.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+
+set -eo pipefail
+
+# How many bytes optipng has to be able to strip out of the file for the optimization to be worth it. The default is 1 KiB.
+: "${MINIMUM_OPTIMIZATION_BYTES:=1024}"
+
+script_path=$(cd -P -- "$(dirname -- "$0")" && pwd -P)
+cd "${script_path}/.."
+
+if ! command -v optipng >/dev/null ; then
+    echo 'optipng is not installed, skipping png size check.'
+    echo 'Please install optipng for your system to run this check.'
+    exit 0
+fi
+
+files=()
+for file in "$@"; do
+    if [[ "${file}" == *".png" ]]; then
+        files+=("${file}")
+    fi
+done
+
+if (( ${#files[@]} )); then
+    # We need to allow optipng to write output so we can check what it actually did. We use a dummy file that's discarded afterwards.
+    optimizations=$( printf '%s\0' "${files[@]}" |\
+        xargs -0 -n1 optipng -strip all -out dummy-optipng-output.png -clobber 2>&1 |\
+        grep -i -e 'Output IDAT size =' |\
+        sed -E 's/Output IDAT size = [0-9]+ byte(s?) \(([0-9]+) byte(s?) decrease\)/\2/g;s/Output IDAT size = [0-9]+ byte(s?) \(no change\)/0/g' |\
+        awk "{ if (\$1 >= $MINIMUM_OPTIMIZATION_BYTES) { S+=\$1 } } END { print S }")
+    rm -f dummy-optipng-output.png dummy-optipng-output.png.bak
+    optimizations="${optimizations:-0}"
+
+    if [[ "$optimizations" -ne 0 ]] ; then
+        echo "There are non-optimized PNG images in Base/. It is possible to reduce file sizes by at least $optimizations byte(s)."
+        # shellcheck disable=SC2016 # we're not trying to expand expressions here
+        echo 'Please run optipng with `-strip all` on modified PNG images and try again.'
+        exit 1
+    fi
+else
+    echo 'No PNG images to check.'
+fi

Meta/lint-ci.sh

@@ -24,6 +24,7 @@ for cmd in \
         Meta/check-debug-flags.sh \
         Meta/check-markdown.sh \
         Meta/check-newlines-at-eof.py \
+        Meta/check-png-sizes.sh \
         Meta/check-style.py \
         Meta/lint-executable-resources.sh \
         Meta/lint-keymaps.py \