Add GitHub workflow for managing downstream WebRender

.github/sync/UPSTREAM_COMMIT

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+816766e1a236fcb7eebaa493e492b55c419ada05

.github/sync/sync.sh

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+#!/bin/sh
+# Usage: sync.sh <path/to/filtered>
+#
+# A script to sync Mozilla's git mirror of WebRender [1] to the Servo project's
+# downstream fork [2]. This script runs as part of a GitHub Action in this
+# repository triggered on regular intervals.
+#
+# The procedure for the sync is:
+#
+# 1. Clone a copy of the GitHub gecko-dev repository to the "_cache" directory.
+# 2. Filter that repository using `git-filter-repo` and create a new local git
+#    repository with the filtered contents into a directory specified by the
+#    argument passed to this script. The filtered contents are determined
+#    by the configuration in `.github/sync/webrender.paths`.
+# 3. Cherry-pick the new commits into the repository in the current working
+#    directory. The commits applied from the filtered repository are determined
+#    by choosing every commit after the hash found in the file
+#    `.github/sync/UPSTREAM_COMMIT`
+#
+# Note that this script relies on the idea that filtering `gecko-dev` the same
+# way more than once will result in the same commit hashes.
+#
+# If at some point, `webrender.paths` is modified and the commit hashes change,
+# then a single manual filter will have to happen in order to translate the
+# hash in the original filtered repository to the new one. The procedure for this
+# is roughly:
+#
+# 1. Run `git-filter-repo` locally and note the new hash of the latest
+#    commit included from upstream.
+# 2. Replace the contents `UPSTREAM_COMMIT` with that hash and commit
+#    it together with your changes to `webrender.paths`.
+#
+# [1]: <https://github.com/mozilla/gecko-dev/> mirrored from
+#      <https://hg.mozilla.org/mozilla-central>
+# [2]: <https://github.com/mozilla/gecko-dev/>
+set -eux
+
+root_dir=$(pwd)
+cache_dir=$root_dir/_cache
+
+# Configure git because we will be making commits.
+git_name="Webrender Upstream Sync"
+git_email="[email protected]"
+
+step() {
+    if [ "${TERM-}" != '' ]; then
+        tput setaf 12
+    fi
+    >&2 printf '* %s\n' "$*"
+    if [ "${TERM-}" != '' ]; then
+        tput sgr0
+    fi
+}
+
+step "Creating directory for filtered upstream repo if needed"
+mkdir -p "$1"
+cd -- "$1"
+filtered=$(pwd)
+
+step "Creating cache directory if needed"
+mkdir -p "$cache_dir"
+cd "$cache_dir"
+export PATH="$PWD:$PATH"
+
+step "Downloading git-filter-repo if needed"
+if ! git filter-repo --version 2> /dev/null; then
+    curl -O https://raw.githubusercontent.com/newren/git-filter-repo/v2.38.0/git-filter-repo
+    chmod +x git-filter-repo
+
+    git filter-repo --version
+fi
+
+step "Cloning upstream if needed"
+if ! [ -e upstream ]; then
+    git clone --bare --single-branch --progress https://github.com/mozilla/gecko-dev.git upstream
+fi
+
+step "Updating upstream"
+branch=$(git -C upstream rev-parse --abbrev-ref HEAD)
+git -C upstream fetch origin $branch:$branch
+
+step "Filtering upstream"
+# Cloning and filtering is much faster than git filter-repo --source --target.
+git clone upstream -- "$filtered"
+git -C "$filtered" filter-repo --force --paths-from-file "$root_dir/.github/sync/webrender.paths"
+
+step "Adding filtered repository as a remote"
+cd "$root_dir"
+git remote add filtered-upstream "$filtered"
+git fetch filtered-upstream
+
+step "Resetting `main` branch to filtered repository HEAD"
+git switch -c __work
+git reset --hard filtered-upstream/master
+git cherry-pick origin/test
+git switch -c test
+git reset --hard __work

.github/sync/webrender.paths

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+# Filters and renames use git-filter-repo(1) --paths-from-file:
+# https://htmlpreview.github.io/?https://github.com/newren/git-filter-repo/blob/docs/html/git-filter-repo.html#_filtering_based_on_many_paths
+#
+# WARNING: Do not modify this file without also updating UPSTREAM_COMMIT. If you
+# do that, it's likely that the sync will fail or add all commits again to the
+# repository. See the documentation in `sync.sh` for more details.
+
+gfx/wr/
+regex:gfx/wr/(.+)==>\1

.github/workflows/sync-upstream.yml

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+name: Sync from mozilla-central
+
+on:
+  schedule:
+    # Midnight on Sunday.
+    - cron: '0 0 * * 0'
+  workflow_dispatch:
+
+jobs:
+  sync:
+    name: Sync
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+      - uses: actions/cache@v4
+        with:
+          path: _cache/upstream
+          key: upstream
+      - name: Run synchronization script
+        run:  ./.github/sync/sync.sh _filtered
+      - name: Pushing new `main`
+        run: git push -f origin test