Add Chromium-only Blender WebEngine parity work

tools/web/build-web-engine-variants.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+source "${repo_root}/tools/web/emscripten-env.sh"
+
+configure_variant() {
+  local name="$1"
+  local threads="$2"
+  local build_dir="${repo_root}/build_web-${name}"
+  emcmake cmake -S "${repo_root}/web/engine" -B "${build_dir}" -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release -DWEB_ENGINE_THREADS="${threads}"
+  cmake --build "${build_dir}" --target web_engine
+  test -s "${build_dir}/web_engine.js"
+  test -s "${build_dir}/web_engine.wasm"
+  printf '%s variant ok: %s\n' "${name}" "${build_dir}"
+}
+
+configure_variant single OFF
+configure_variant pthread ON