Workflow for updating CImg (#137)

Also updates CImg to 3.3.3... huh, coincidence? <!-- I think not -->

.github/workflows/cimg.yml

@@ -0,0 +1,43 @@
+name: Update CImg
+on:
+  schedule:
+    - cron: 0 0 3 * * # The first day after this workflow was merged
+  release:
+    types: [published]
+  workflow_dispatch:
+jobs:
+  check-version:
+    name: Check for updates
+    runs-on: ubuntu-latest
+    outputs:
+      latest-tag: ${{ steps.latest-tag.outputs.result }}
+    steps:
+      - name: Fetch the latest tag (could be buggy)
+        uses: actions/github-script@v7
+        id: latest-tag
+        with:
+          result-encoding: string
+          script: |
+              return (await github.rest.repos.listTags({ owner: 'GreycLab', repo: 'CImg', per_page: 28 })).data[27].name
+  pull-file:
+    name: Update CImg.h
+    runs-on: ubuntu-latest
+    needs: check-version
+    if: ${{ needs.check-version.outputs.latest-tag != 'v.3.3.3' }}
+    steps:
+      - uses: actions/checkout@v4
+        name: Checkout this repository
+        with:
+          token: ${{ secrets.CIMG_UPDATE_TOKEN }}
+      - name: Download new CImg version
+        uses: carlosperate/download-file-action@v2
+        with:
+          file-url: 'https://github.com/GreycLab/CImg/raw/${{ needs.check-version.outputs.latest-tag }}/CImg.h'
+          location: 'src'
+      - name: Commit new CImg version (and update script)
+        run: |
+          sed -i 's/v.3.3.3/${{ needs.check-version.outputs.latest-tag }}/' .github/workflows/cimg.yml
+          git config user.name 'GitHub Actions'
+          git config user.email 'actions@github.com'
+          git commit -am "Update CImg to ${{ needs.check-version.outputs.latest-tag }}"
+          git push