Required for making Reactor thread safe.

Bug: b/153803432
Change-Id: I6d36346e22e4ba7a629e280baaa8d293969c366f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44012Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>