CI: check that all png images are on git LFS (#5321)

To prevent egui repository from growing forever

* Sibling PR: https://github.com/emilk/egui/pull/5320

.github/workflows/png_only_on_lfs.yml

@@ -0,0 +1,23 @@
+name: All; .png on git LFS
+
+on: [push, pull_request]
+
+jobs:
+  check-binary-files:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    - name: Check that png files are on git LFS
+      run: |
+        binary_extensions="png"
+
+        # Find binary files that are not tracked by Git LFS
+        for ext in $binary_extensions; do
+          if comm -23 <(git ls-files | sort) <(git lfs ls-files -n | sort) | grep "\.${ext}$"; then
+            echo "Error: Found binary file with extension .$ext not tracked by git LFS. See CONTRIBUTING.md"
+            exit 1
+          fi
+        done