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

To prevent egui repository from growing forever

* Sibling PR: emilk#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

CONTRIBUTING.md

@@ -35,9 +35,11 @@ You can test your code locally by running `./scripts/check.sh`.
 There are snapshots test that might need to be updated. Run the tests with `UPDATE_SNAPSHOTS=true` to update them.
 For more info about the tests see [egui_kittest](./crates/egui_kittest/README.md).
 
-We use [git-lfs](https://git-lfs.com/) to store big files in the repository. 
+We use [git-lfs](https://git-lfs.com/) to store big files in the repository.
 Make sure you have it installed (running `git lfs ls-files` from the repository root should list some files).
 Don't forget to run `git lfs install` after installing the git-lfs binary.
+You need to add any .png images to `git lfs`.
+If the CI complains about this, make sure you run `git add --renormalize .`.
 
 If you see an `InvalidSignature` error when running snapshot tests, it's probably a problem related to git-lfs.