Thread pinning

[randy408]

Describe the problem you are trying to solve.
Pinning systems to threads is important to keep specific library calls on the main thread, prevent splitting of a module's systems across threads and to allow thread-safe systems to run on any thread at the same time.

Describe the solution you'd like

* Updated *

• Introduce the concept of a main flecs thread which is the ecs_progress() thread / thread index 0 as far as flecs is concerned.

• For backward compatibility existing systems should run on the main thread unless specified otherwise

• With the exception of the main thread, thread pinning should be always to be to a pseudo-thread, the flecs thread index could change at any time as long as the pinned systems don't run on different threads, this only has to be a specification at first.

• For systems that are threadsafe there should be an easy way to mark them as such

// Always run on thread idx 0 by default
ECS_SYSTEM("Move", "Position, Velocity");

//Mark system as thread-safe, alternative names: MultiThread, AllThreads, AnyThread
ECS_SYSTEM("X", "SYSTEM:ThreadSafe Lorem, Ipsum");

//Creates an entity
ECS_THREAD(MotionThread);

//"Thread" is a builtin trait, makes it more obvious `MotionThread` is a thread
//System is not split across threads, but the thread index may change
ECS_SYSTEM("Move", "SYSTEM:Thread FOR MotionThread, Position, Velocity");

//Also not split across threads and on the same thread as Move
ECS_SYSTEM("X", "SYSTEM:Thread FOR MotionThread Lorem, Ipsum");

//Alternative syntax
ECS_SYSTEM("Move", "THREAD:MotionThread Position, Velocity");

Why something like THREAD:0-N would be a bad idea:
Modules from different sources could hog the same thread and assumes flecs will spawn N threads, named threads are also better for debugging.

[alex-rass-88]

Have some question about threading. Can i create custom pipeline with some stages marked as single threaded? I need 2 stages what runs on main thread and 5 stages what multi threaded.