This can be used to switch to a specific engine.
If the state machine is not running in the passed engine,
then it performs a yield to that engine, otherwise it keeps
running. Hence, putting this at the top of a state
guarantees that it runs in that engine.
For example:
case FrontEnd_done:
{
// Unlock must be called by the same thread that locked it.
if (yield_if_not(&gStateMachineThreadEngine))
{
break;
}
// Here we are running in gStateMachineThreadEngine.
...
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