Merge SDL_CreateThread

[icculus]

Merge SDL_CreateThread and SDL_CreateThreadWithStackSize into one function, remove the hint for specifying stack size.

(And, maybe, if we add thread affinity, add that to SDL_CreateThread too, but that can come later.

[slouken]

If we add thread affinity too, that needs to come before ABI stabilizes in 3.2.0.

[icculus]

I'm assign this and #6636 to myself for later, but it's going to be a bit before all the SDL3 migration fires are out and I can actually write new code. :)

[slouken]

Remind me again why we need to do this? Most people don't need to specify the stack size, so it seems reasonable to leave the simpler entry point in the Simple DirectMedia Layer API :)

[icculus]

This was just a note about how we tacked on a second API in SDL2 to fill a need, but if it makes sense to keep a more complex thing separate, I'm fine with that.

I'm not sure thread affinity is worth adding at this time, either, honestly.

[slouken]

Okay, let's go ahead and close this for now.