Added function to check the state of a thread

[Mori-TM]

Description

SDL_GetThreadState returns the current state of a thread as enum SDL_ThreadState, which is now also accessible for the user.

• Added SDL_GetThreadState
• Moved SDL_ThreadState from SDL_thread_c.h -> SDL_thread.h
• The "available since" version might not be correct on the new function and the old enum

Existing Issue(s)

#11702

[slouken]

This seems good to me, though I might remove _STATE from the enum values. @icculus, thoughts?

[icculus]

Agreed, let's drop _STATE. But otherwise, I'm on board.

[icculus]

The \since should be updated, as the enum wasn't in the public api before this.

But I'm not sure what the right version is, since we might do a release candidate before 3.2.0. Let's fix that after merging the PR.

[slouken]

The version should be 3.1.8

[slouken]

I'll go ahead and merge, and we can fix it up afterwards. Thanks!