Add workflow to accept snapshots via kitdiff (#7577)

This adds a new workflow `update_kittest_snapshots.yml` that can be
triggered through the [kitdiff](https://github.com/rerun-io/kitdiff) ui
when viewing a ci artefact.
Also adds a link to kitdiff to view the pr changes to each commit (via
the preview build comment)

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.github/workflows/preview_build.yml

@@ -17,6 +17,20 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
+
+      - name: Comment PR
+        uses: thollander/actions-comment-pull-request@v2
+        env:
+          URL_SLUG: ${{ github.event.number }}-${{ github.head_ref }}
+        with:
+          message: |
+            Preview is being built...
+            
+            Preview will be available at https://egui-pr-preview.github.io/pr/${{ env.URL_SLUG }}
+            
+            View snapshot changes at [kitdiff](https://rerun-io.github.io/kitdiff/?url=${{ github.event.pull_request.html_url }})
+          comment_tag: 'egui-preview'
+
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
         with:

.github/workflows/preview_deploy.yml

@@ -61,5 +61,7 @@ jobs:
           message: |
             Preview available at https://egui-pr-preview.github.io/pr/${{ env.URL_SLUG }}
             Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.
+            
+            View snapshot changes at [kitdiff](https://rerun-io.github.io/kitdiff/?url=${{ github.event.pull_request.html_url }})
           pr_number: ${{ env.PR_NUMBER }}
           comment_tag: 'egui-preview'

.github/workflows/update_kittest_snapshots.yml

@@ -0,0 +1,40 @@
+on:
+  workflow_dispatch:
+    inputs:
+      run_id:
+        description: 'The run ID that produced the artifact'
+        required: true
+        type: string
+
+permissions:
+  actions: read
+
+name: Update kittest snapshots
+jobs:
+  update-snapshots:
+    name: Update snapshots from artifact
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          lfs: true
+          # We can't use the workflow token since that would prevent our commit to cause further workflows.
+          # See https://github.com/stefanzweifel/git-auto-commit-action#commits-made-by-this-action-do-not-trigger-new-workflow-runs
+          token: '${{ secrets.GITHUB_TOKEN }}'
+
+      - name: Accept snapshots
+        env:
+          GH_TOKEN: ${{ github.token }}
+          RUN_ID: ${{ github.event.inputs.run_id }}
+        run: ./scripts/update_snapshots_from_ci.sh
+
+      - name: Git status
+        run: git status
+
+      - uses: stefanzweifel/git-auto-commit-action@v6
+        with:
+          commit_message: 'Update snapshot images'
+