Add Chromium-only Blender WebEngine parity work

blender-5.2.0/source/blender/blenlib/BLI_mutex.hh

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+/* SPDX-FileCopyrightText: 2025 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+/* Always include that so that `BLI_mutex.hh` can be used as replacement to including <mutex>.
+ * Otherwise it might be confusing if both are included explicitly in a file. That also makes the
+ * difference between compiling with and without TBB smaller. */
+#include <mutex>  // IWYU pragma: export
+
+#ifdef WITH_TBB
+#  include <tbb/mutex.h>
+#endif
+
+namespace blender {
+
+#ifdef WITH_TBB
+
+/**
+ * Mutex should be used as the default mutex in Blender. It implements a subset of the API
+ * of std::mutex but has overall better guaranteed properties. It can be used with RAII helpers
+ * like std::lock_guard. However, it is not compatible with e.g. std::condition_variable. So one
+ * still has to use std::mutex for that case.
+ *
+ * The mutex provided by TBB has these properties:
+ * - It's as fast as a spin-lock in the non-contended case, i.e. when no other thread is trying to
+ *   lock the mutex at the same time.
+ * - In the contended case, it spins a couple of times but then blocks to avoid draining system
+ *   resources by spinning for a long time.
+ * - It's only 1 byte large, compared to e.g. 40 bytes when using the std::mutex of GCC. This makes
+ *   it more feasible to have many smaller mutexes which can improve scalability of algorithms
+ *   compared to using fewer larger mutexes. Also it just reduces "memory slop" across Blender.
+ * - It is *not* a fair mutex, i.e. it's not guaranteed that a thread will ever be able to lock the
+ *   mutex when there are always more than one threads that try to lock it. In the majority of
+ *   cases, using a fair mutex just causes extra overhead without any benefit. std::mutex is not
+ *   guaranteed to be fair either.
+ */
+using Mutex = tbb::mutex;
+
+/* If this is not true anymore at some point, the comment above needs to be updated. */
+static_assert(sizeof(Mutex) == 1);
+
+#else
+
+/** Use std::mutex as a fallback when compiling without TBB. */
+using Mutex = std::mutex;
+
+#endif
+
+}  // namespace blender