Use ThreadWorkPool instead of thread_process_array in NavMap [3.x]

[adamscott]

3.x PR of #60359

Finally, porting PR #60359 to 3.x was easier than anticipated. All because core/templates/thread_work_pool.{h, cpp} was easily ported (only required some copy and paste).

Note. This PR seems to work, but I do not know if I broke some rules, as core/templates didn't already exist.

As a benchmark, I used the minimal reproduction project of issue #56838 made by thoced.
NavigationAgentTest.zip

Godot 3.x [master] (using thread_process_array), first minute, VSYNC disabled (Windows 10)

Godot 3.x [master] (using ThreadWorkPool), first minute, VSYNC disabled (Windows 10)

As the addition of the new template, this pretty much confirms that a simple cherry-pick of #60359 wouldn't work for 3.x

Along with PR #60359, fixes #56838. But this needs testing.

[akien-mga]

Please add thread_work_pool.h in core/os/ with the other Thread related functionality in 3.x, instead of a templates. The core structure changed in master but for 3.x we should keep the pre-existing structure.

[RandomShaper]

As discussed on IRC, the engine should be improved in regards to thread pooling (namely, removing threaded_array_processor() and revising thread pooling so that pools are managed in a more automatic and rational way, depending on platforms. etc.).

We've agreed to let this in anyway, since it's already an improvement given the current state of things, while the deeper discussion comes.

[adamscott]

Please add thread_work_pool.h in core/os/ with the other Thread related functionality in 3.x, instead of a templates. The core structure changed in master but for 3.x we should keep the pre-existing structure.

I moved the files as asked! Thanks!

[adamscott]

Backported merged PR #60528 made by @bruvzg to init step_work_pool only when needed.

[akien-mga]

Thanks!