Add images size checking in PR (#281)

* Add images size checking in PR

* Fix typo

* Switched to POSIX shell

* More proper diff

.github/workflows/pr.yaml

@@ -0,0 +1,12 @@
+on: pull_request
+
+jobs:
+  check_size:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Check file size
+        run: .github/workflows/scripts/check-size.sh
+

.github/workflows/scripts/check-size.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+set -eu
+
+SIZE_LIMIT=150000
+
+check_size() {
+    local size=$(stat --printf="%s" $1)
+    if [ "$size" -gt "$SIZE_LIMIT" ]; then
+        echo "File $1 is bigger than specified $SIZE_LIMIT limit"
+        exit 1
+    fi
+}
+
+git diff --name-only `git merge-base origin/master HEAD` | while IFS= read -r file; do
+    case "$file" in
+        *.webp)
+            echo "Checking size of $file"
+            check_size $file
+            ;;
+        *)
+            echo "Skipping $file"
+            ;;
+    esac
+done
+